LECTURE NOTES ON INVERTEBRATE ZOOLOGY essay

LECTURE NOTES ON INVERTEBRATE ZOOLOGY

SZL 111: INVERTEBRATE ZOOLOGY

LECTURE I: INTRODUCTION

Biology is the study of living things, which can be identified by the following characteristics. 1. Respiration 2. Nutrition 3. Excretion 4.  Reproduction. 5. Response 6.  Growth 7. Locomotion.

They are divided into 5 taxonomic groups called kingdoms.

1. Kingdom Monera (Prokaryotes)

Members of this kingdom have no nuclear membrane holding genetic materials. They include Archenobacteria. Eubacteria and Cyanobacteria (blue-green algae.

2. Kingdom Pritista (Pritoctista)

The kingdom consists of protozoa and unicellular algae. Members of this kingdom have a membrane bound nucleus that confines complex chromosomes. I.e. they are eukaryotic.

Members of these two kingdoms may be autotrophs (self-nourishing, synthesizing their own food from inorganic constituents with the aid of solar or chemically derived energy). They may also be heterotrophs (being nourished by others) obtaining their food from the bodies of other organisms. They may also be both.

3. Kingdom Fungi

These are multicellular saprophytic heterotrophs. Examples of members of this kingdom include mushrooms, yeasts, and molds. They take up nutrients from organic matter, either alive or decomposing.

4. Kingdom Plantae

This kingdom is made up of photosynthetic autotrophs. It consists of the green plants.

5. Kingdom Animalia

The kingdom consists of ingestives which live by eating other organisms. They obtain, digest and assimilate food. They circulate nutrients and gases within the body. They dispose metabolic wastes. They coordinate activities, often with both nervous and endocrine systems. They avoid unpleasant environments, grow and reproduce.

Biological classification

This is a system of grouping individuals in an organized manner that reflects similarities and differences among them.

There are various categories called taxons, which are used in grouping the animals. In the animal kingdom, taxons are arranged with phylum being the highest category and the taxons at successively lower levels containing animals that are increasingly closely related and presumably similar.

At the lowest level of the system is the ’species’, which is a population of individuals that are able to interbreed and produce fertile offspring and that are reproductively isolated.

The categories are as follows:

Kingdom

Phylum

Class

Order

Family

Genus

Species.

All the individuals in a species are morphologically similar enough to each other that they can be recognized as an interbreeding unit with genetic exchange.

Challenges faced in animal classification

1. Sexual polymorphism and caste system: Sometimes individuals in a species are markedly different e.g. with dimorphic males and females, adults and immature forms; caste system in social insects.
2. Synonymy: An animal species identified by different taxonomists may be given different generic and species names.
3. Sibling species: Closely similar individuals are recognized as belonging to different species only after detailed observations and genetic analyses have been made.
4. Seasonal polymorphism: Some organisms may change their forms and colors with seasonal variation and may be mistaken for different species.

As biologists increase their ability to discriminate differences, more species are recognized. Closely related species are grouped together into taxonomic units called ‘genera’. Genera that are similar to each other are grouped together into taxonomic units called ‘families’. Families are then grouped together into orders, orders into classes and classes into phyla. Generic and species names are underlined or printed in italics.

Classification and evolution

Species within each taxonomic unit are assumed to be descendants of a common ancestor and all of the descendants of that common ancestor are grouped together so that each taxonomic unit is monophyletic.

Classification system within a phylum is an attempt to portray evolutionary relationships.

LECTURE 2: SUB-KINGDOM PROTOZOA

General Characteristics

Heterotroph
Most motile
Eukaryotes
Unicellular

Nutrition Protozoans hunt, digest and store food. Most are heterotrophs.  They feed on other animals to obtain the nutrients they need to live.

Movement Protozoans move by the use of cilia, flagella, pseudopods or some have no movement (Sporozoa).

Reproduction Asexual reproduction of protozoans occurs when the cell divides in half by binary fission.  Some which are parasites multiply within the host.  Some protozoans reproduce sexually as well.  This can happen when two protozoans carrying half of their regular genetic material fuse together and form a new cell.  Others exchange genetic material during mating.

Respiration takes place when oxygen diffuses into the cell, where the food molecules become oxidized.  The energy produced and the organic molecules are used for maintenance and building of the cell.  Waste products, carbon dioxide and water diffuse out of the cell.

Excretion Waste materials in protozoans are removed from the cell by diffusion through the cell.  They are transported out of the cell by food vacuoles that come in contact with the surface.  This is known as exocytosis.

Protozoa classification

Protozoans are classified into four phyla: Phylum Sarcodina, Phylum Ciliophora, Phylum Sporozoa and Phylum Mastigophora.

Phylum Sarcodina (Rhizopods)

Phylum Sarcodina (Rhizopods) are the largest phylum of protozoans with about 18,000 known species.

Some Amoebas (Rhizopods) are harmless, while others cause serious disease (Entamoeba histolytica)

Amoebas are constantly changing their shape.

Some amoebas are as small as 0.25mm in length while others can be up to 8mm.

Pseudopods may be rounded at the tip (lobopodia), pointed (filopodia), branched and fused together (rhizopodia) or somewhat rigid and pointed (axopodia).

General Characteristics

Nutrition They engulf their food with their pseudopods where it then forms a food vacuole. This is where the food is broken down and spread throughout the rhizopod. This process is called phagocytosis.

Movement They move with the use of their pseudopods.

Reproduction Amoebas, under favorable conditions, reproduce by binary fission (This means dividing in two), producing two daughter amoebas. The nucleus divides by mitosis.  If ameobas are divided artificially the nucleus does not divide.  Only the half of the amoeba containing the nucleus survives while the other half dies.  If conditions are unfavorable the amoeba will secrete a protective covering and wait until conditions are favorable to divide.

Excretion Materials that are useless to the amoeba are held in the food vacuole until the vacuole comes in contact with the membrane of the body surface. This is where the waste is expelled from the body

Respiration Respiration takes place by diffusion of gases through the cell membrane

Defense The Arcella, known as a domed Amoeba, builds a transparent dome from whatever building materials it can find in the pond, usually sand. It then encloses itself in the dome for protection.

Economic Importance

The skeletons of Foramininfera make up much of the limestone and chalk on the Earth.

Entamoeba histolytica lives in the human large intestine and attacks the intestinal wall with enzymes, causing severe and often fatal diarrhea.

1. Entamoeba coli in the intestine and E. gingivalis in the mouth are not disease agents.
2. Some rhizopods have a siliceous or chitinoid test for protection; pseudopodia project from openings.

Examples of Rhizopods

Amoeba proteus (common amoeba)
Entamoeba histolytica (causes amoebic dysentery)
Radiolarians (internal glass-like skeletons)
Foraminifera (large shelled amoeboid belonging to Sarcodina)

Phylum Ciliophora (Ciliates)

General Characteristics

Ciliates have a skeleton like covering made of Polysaccharides.

Method of movement. It swims rapidly by coordinated wavelike beats of its many cilia.  A paramecium normally moves forward in a corkscrew fashion but is capable of reversing direction when it encounters undesirable conditions.

Reproduction

The larger nucleus is thought to regulate cell functions, while the smaller nucleus is involved in reproduction.  Ciliates usually reproduce asexually by binary fission. Sometimes they reproduce sexually. This takes place when two individual ciliates join together at the oral grooves and exchange portions of the micronuclei. After this stage each individual divides. Paramecia can divide as often as two or three times a day.

Nutrition At one side of the ciliates body there is a mouth like opening called the oral groove. The food is swept into the oral groove by the cilia.  Food consists of other protozoans, bacteria and algae.

Excretion Undigested food is discarded through the anal pore (which is some distance behind the oral groove).  Metabolic wastes diffuse out directly at the body surface.

Respiration Respiration of Ciliates can be anaerobic (occurring without oxygen) or aerobic (requiring oxygen).  Anaerobic protozoa must live without oxygen and will die if exposed to it.  Respiration rate varies directly with temperature and also depends on the kind of molecules broken down for energy.   It also varies with species.

Defense In many ciliates there is a layer of many tiny carrot shaped bodies, called Trichocysts, underneath the pellicle.  When discharged each Trichocysts forms a long sticky thread.  Usually they are discharged as a means of defense but are sometimes used as an anchor.

Examples of Ciliates

Paramecuim (Paramecium caudatum), Stentor (Stentor coeruleus), Balantidium coli

Economic importance of ciliates

Balantidium is the only ciliated protozoan known to infect humans. Balantidiasis is a zoonotic disease and is acquired by humans via the feco-oral route from the normal host, the pig, where it is asymptomatic. Contaminated water is the most common mechanism of transmission.

Phylum Sporozoa (Apicomplexa)

General Characteristics

1. Not Motile
2. All are endoparasites; hosts are in many animal phyla.
3. An apical complex is a feature of this phylum; it is present only in certain stages.
4. Rhoptries and micronemes help it penetrate the host’s cells.
5. Pseudopodia occur in some stages; gametes may be flagellated and contractile fibrils may form waves to propel it through liquid.
6. The life cycle usually includes both sexual and asexual stages; an invertebrate may be an intermediate host.
7. At some point, they form a spore (oocyst) that is infective in the next host and protects the sporozoan.

Method of Movement Sporozoans have no physical form of movement. However, they can be moved by the currents of the blood or other fluids of their hosts.

Reproduction They reproduce sexually in one host and then asexually in a second host.

Nutrition They are parasites.  Sporozoans have special organelles that allow it to invade a host cell.

Defense The Plasmodium vivax (Sporozoan) tucks inside the red blood cells, where the parasites are protected from attack by the antibodies of the host. Even when they leave to invade fresh cells, they evade immune destruction by constantly changing their antigens [any of several subsatnces that cause the development of antibodies].

Examples of Sporozoans

Plasmodium vivax (Causes Malaria)
Monocystis agilis

Class Sporozoea

1. Sporozoea is the most important class; it contains three subclasses.

1)         Gregarinia, or gregarines, are common parasites of invertebrates but are of little economic import.

2)         Piroplasmia includes some veterinary parasites: Babesia bigemina causes Texas red-      water fever in cattle.

3)         Coccidia are important intracellular parasites in both invertebrate and vertebrates.

            Eimeria is a genus (along with Isospora) that causes coccidiosis.

1)         Isospora infections are mild unless the immune system is weak, as in AIDS patients.

2)        Eimeria tenela is often fatal to young fowl.

3)         Organisms undergo schizogony in intestinal cells; the zygote forms an oocyst that passes in the feces and releases eight sporozoites when ingested by the next host.

Economic importance of Apicomlexa

Plasmodium: The Malarial Organism

1)         Malaria is the most important infectious disease of humans.

2)         Four species infect humans; each produces different clinical symptoms.

3)         Anopheles mosquitoes carry all forms; the female injects the Plasmodium in her saliva.

4)         Sporozoites penetrate liver cells and initiate schizogony.

Sporozoa are the only animal like protists that have no form of movement.

The plasmodium (Sporozoan) causes malaria.

Sporozoa need both a human or other vertebrate and a mosquito to complete their life cycle.

Phylum Mastogophora (Flagellates)

General Characteristics

Motile

Method of movement They move with the use of Flagella an example is a Trypanosome

Nutrition They get their nutrients by eating other organisms or by absorbing food molecules through cell membranes.

Defense Members of the Trypanosoma Genus releases a poisonous substance that attacks the nervous system, causing weakness and eventually death (African sleeping sickness).

Excretion Undigested wastes leave trough the anal pore. Contractile vacuoles are used in pumping water out of the cell.

Economic Importance Subphylum Kinetoplasta

1. Zooflagellates lack chromoplasts and have holozoic or saprozoic nutrition; most are symbiotic.

b.Trypanosoma is an important genus of protozoan parasites; some are not pathogenic.

1)   Trypanosoma brucei gambiense and T. b. rhodesiense cause African sleeping sickness in humans.

2)   T. brucei brucei causes a related disease in domestic animals.

3)   These trypanosomas are transmitted by tsetse flies; natural reservoirs include antelope and other wild mammals.

Flagellates are the primary component in the marine food chain

4)   Half of the 10,000 new cases each year are fatal; the remainder may suffer brain damage.

5)   Trypanosoma cruzi causes Chagas disease in Central and South America; this parasite is carried by a bug and causes nervous system problems.

1. Leishmania species cause visceral diseases in humans; they are transmitted by sand flies.
2. Another common protozoa is Trichomonas, a sexually transmitted flagellate that can     cause urogenital symptoms in infected women. Various species of Trichomonas live in the cecum, colon, mouth and urogenital tracts of humans.
3. Giardia is a common pathogenic flagellate that causes diarrhea and is known informally as Beaver Fever.

LECTURE 3: PHYLUM PORIFERA

Introduction

Sponges are regarded as most primitive metazoans with lower grade of body organizations. Body of sponges is made of cells, no tissues or organs found in their body. They are sedentary animals and look like plants. They can be differentiated from the protozoans in having cells and skeleton in the forms of spicules.

A group of metazoan animal whose bodies are without any symmetry or radially symmetrical, without mouth and nervous system and whose bodies are provided with many pores, canals and chonaocyte cells, are recognized as Porifera or sponges.

Body skeleton, i.e., spicules and sponging fibres are the basis of the classification of Porifera.

General Characters

• All sponges are aquatic, marine or freshwater; remain attached to some submerged substratum.
• Body flower-vase like or tubular, with radial symmetry or without any symmetry.
• Multicullular body is provided with many pores or Ostia.
• The space in between ectoderm and endoderm is filled with mesenchyme, i.e., the animal is diploblastic.
• Body is having canal system.
• Flagellated cells or choanocytes line the radial canal.
• Internal skeleton is made up of spongin fibre, siliceous or calcareous spicules.
• Reproduction sexual or asexual, asexual reproduction takes place by the formation of gemmules.

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• Sponges have no nervous system or sensory cells, possess no mouth, proper tissue or organs.
• The cells of the body are capable of de-differentiation i.e. reverting to an undifferentiated condition but they may then differentiate again into any type of cell found in the body.

Reproduction in Porifera

• Besides reproducing asexually by budding and regeneration, sponges also reproduce sexually by the formation of germ cells. These are developed from the amoebocytes.
• Fertilization is followed by cleavage and then a peculiar larva is formed. This is the amphiblastula larva. This is oval in shape, with one half covered by small flagellated cells and the other half by larger granular cells.
• After swimming freely for some time the amphiblastula settles down to develop into the sedentary adult sponge.

Phylum – Porifera

The phylum porifera is divided into three classes; Class Calcarea, Class Hexactinellida and Class Demospongiae.

Class                                       Subclass

1. Calcarea                                           (i) Calcaronea

(ii) Calcinea

1. Hexactinellida                           (i) Hexasterophora

(ii) Amphidiscophora

III. Demospongia                           (i) Tetractinomorpha

(ii) Ceractinomorpha

Class I- Calcarea

Characters:-

1. All are marine, single in live in colony.
2. Endoskeleton is composed of spicules made of calcium carbonate.
3. Choanocyte cells are large.
4. Osculum small surrounded by spicules.

Class Calcarea has been divided into two Sub Classes:

Sub Class (i) Calcaronea

Characters:-

1. Nucleus of the choanocyte lies at the base of the collar.
2. Flagellum arises from the basal granule attached with the nucleus.
3. Spicules triaxon or 3-rayed, one ray is larger.

Examples- Ascon type (= Scypha), Leucosolenia

Genus Leucosolenia

Leucosolenia is a simple sponge which grows on rocks near the seashore in colonies. It has a skeleton of calcium carbonate, deposited mostly in the form of minute triradiate spicules.

External Features

• General form of colony, with numerous horizontal branches over which sprout many vase-shaped individuals, each with a single large opening at its free end, the osculum.
• The body wall is thin, transparent, perforated by many tiny incurrent pores or Ostia and supported by numerous triradiate spicules which lie in the wall with two rays towards the osculum and one way from it.
• In live specimens a continuous current of water passes in through the Ostia and out through the osculum.

Internal Structures

The body wall is simple, very thin and formed of two layers:

• The dermal layer lies on the outside and is formed of an outer covering layer of flattened cells or pinacocytes, and inner layer of numerous scattered cells embedded in a non-living gelatinous matrix.

Cells of the dermal layer

Scleroblasts are the most numerous of all and secrete needle like calcareous spicules in their protoplasm.

Amoebocytes are wandering cells in the jelly and can develop into any of the more specialized cells in the body.

Porocytes are conical cells, each pierced by a small central tube that acts as incurrent pore (Ostia)

• The spongocoel layer lies to the inside, lining the whole of the spongocoel cavity.

Spongocoel cells

The spongocoel is composed of a single layer of collared flagellated cells called choanocytes. The cells stand side by side, do not touch, and the free end of each cell bears a single flagellum, encircled at its base by a delicate protoplasmic collar.

Sub Class (ii) Calcinea

Characters:-

1. Nucleus of the chonocyte lies at the base of the collar.
2. Flagellum does not originate from the nucleus.
3. Spicules tri-rayed, all rays are equal.

Examples- Clathrina, Petrobiona

Class II. Hexactinellida

Characters:-

1. Flowre-vase like body.
2. Spicules are composed of silica.
3. Choanocytes are restricted to flagellated chamber.

Class II. Hexactinellida has been divided into two Sub classes:

Sub Class (i) Hexasterophora

Characters:-

1. Spicules are smaller, triaxon and sixrayed.
2. Spicules with pointed end.

Example: Euplectella

Sub Class (ii) Amphidiscophora

Characters:-

1. Spicules comparatively large.
2. Both end of the spicule is provided with disc like plates.

Example: Hyalonema

The Sycon Type

Genus Sycon

• It is a solitary sponge that lives attached to rocks near the seashore and possesses a skeleton of calcareous spicules.

External Features

• The general body form is vase –shaped, with a large osculum at the free end, and encircled by a fringe of large straight monoaxonic spicules.
• Buds may be seen arising from the base.
• The body wall is thick and perforated by numerous Ostia with spicules projecting around them, giving the animal a bristly appearance.

Internal Structure

• The spongocoel is large and lined with ordinary pinacocytes, not collared cells.
• The body wall has essentially the same structure as that seen in Leucosolenia but is regularly thrown up into numerous radial thimble-shaped flagellated chambers which are lined with collared cells, open into the spongoceol and have their walls pierced by numerous intracellular pores of the porocytes.
• Incurrent canals, lined with pinacocytes, run between these chambers and open on to the surface by minute Ostia.
• In live specimens, water enters by Ostia into the inhalant canals, passes through the pores into the flagellated chambers, from there into the spongoceol and finally passes out through the osculum.

Class III. Demospongia

Characters:-

1. Endoskeleton may be formed by siliceous spicules or sponging fibres or both.
2. Choanaocytes are restricted to spherical chambers.
3. Spicules are having one to four rays.

Class Demospongia has been divided into two Sub Classes as:

Sub Class (i) Tetractinomorpha

Characters:-

1. Spicules tetraxon types.
2. No spongin fibre, only siliceous spicule present.
3. Radially symmetrical body.

Examples: Cliona, Poterion

Sub Class (ii) Ceractinomorpha

1. Endoskeleton made of spongin fibres only.
2. Siliceous spicules when present  monaxon type with one ray only.

Examples: Spongilla, Helichondra

Hartman and Goreau (1970) proposed addition of one more Class Sclerospongiae based on their researches on Jamaican sponges (Barnes- 1980). The characters of the Class are-

1. Endoskeleton made of siliceous spicules and spongin fibres.
2. Exoskeleton made of calcareous spicules.
3. Star shaped canals system.
4. Many Oscula.

The Leucon Type

Genus Euspongia (The bath sponge)

• The bath sponge, like the vast majority of sponges, belongs to the leucoid type or the most complex type of the sponge in which there is a further increase in the folding of the body wall.
• This results in the formation of a very complex system of canal, the invagination of the choanocyte layer into the spongocoel.
• The numerous minute Ostia scattered on the surface lead into extensive hypodermal cavities and branching incurrent canals which open into spherical flagellated chambers.
• Larger excurrent canals lead out from the flagellated chamber and collect to form a somewhat branched and comparatively small spongocoel that opens onto the surface by several oscula.
• Different species of Euspongia live on rocky sea-bottoms, fixed to the substratum by a secretion of sponging, a horny substance of which the skeleton is formed.

1. Beneficial sponges

Sponges are beneficial to mankind and other animals in the following ways.

• As food: crustaceans are found leading parasitic life on them and some depend on upon sponges for their diet.
• As commercial:Sponges serve as protective houses for several animals like crustaceans, worms, molluscs, small fishes, etc., because their enemy cannot feed the sponges. In addition to the protection, the animals living inside the sponge body get a rich food supply from the water circulating through them.
• The dried, fibrous skeleton of many sponges likeSpongia, Hippospongia,and Euspongiaare used for the purpose of bathing, polishing, washing cars, walls, furniture, scrubbing a floor, etc.
• The skeleton of some sponges like Euplectella are of great commercial value and used as decorative pieces.

1. Harmful sponges

• Sponges may cause the death of some sessile animals by growing over them and cutting off food and oxygen supply.
• The boring sponges, like Cliona, attach themselves to the shells of oysters, clams, and branches, etc. It bores into the shells of these animals and completely destroyed them.
• The boring sponges also cause great harm to oyster beds.
• The boring sponges also destroy rocks by penetrating into them and breaking them into pieces.

LECTURE 4: PHYLUM CNIDARIA OR COELENTERATA

                        PHYLUM CTENOPHORA

• This phylum and all the following phyla belong to the subkingdom Metazoa.
• The Cnidarians show a very distinct advance in structure over the porifera.
• Their cells are much more specialized, with a higher co-ordination than in sponges, maintained by a simple polarized nervous system in the form of a network.
• Similar cells, therefore work together to perform a common function, thus beginning to form tissues. In other words, the Cnidaria have reached the tissue grade of organization.
• They are radially symmetrical animals, mostly marine, solitary or colonial and sedentary or free swimming.
• They are also diploblastic i.e. their body is built up of two cellular layers only, an outer, ectoderm and an inner endoderm.
• In between the two layers, there is a jelly like structure-less mesoglea.
• There is a single cavity to the body, the enteron (coelenterons) or gastro-vascular cavity with one opening to the exterior, the mouth which at the same time acts as the anus.
• They possess peculiar elaborate defensive structures, the nematocysts which may be also used in capturing their prey.
• Polymorphism, alternation of generations and skeleton formation are common phenomena in this phylum.

Classification of Cnidaria

The phylum is divided into three classes; Hydrozoa including the freshwater polyps, small jellyfishes and hydroids; the Scyphozoan (Scyphomedusae) comprises of the large jellyfishes and Anthozoa (Actinozoa) including the flower-like sea anemones and most of the stony reef-building corals.

1. Class Hydrozoa

The hydrozoans comprise solitary and colonial forms, among the latter being reef-building corals.

They often show an alternation of sexual hydroid and sexual medusoid generations although either generation may be reduced or eliminated.

1. Hydra, Order Hydroida

• This genus belongs to the order Hydroida and is peculiar in being a common inhabitant of fresh water ponds and slow streams, from which is procured for laboratory studies.
• The medusoid stage is absent while the hydrozoid stage is solitary and sedentary, although capable of detaching itself from the substratum from time to time to change places.
• Hydra is about 4 to 10 mm in length and feeds on small crustaceans, such as Daphnia and Cyclops, and some other small animals.
• Three species are common: vulgaris, H.fusca and H.viridis.
• Symbiotic zoechloreline live in the endodermal cells of the last species, giving the animal its green coloration.

1. Body form

Cylindrical with a crown of tentacles (six to eight) on the free end; the other end, at which the body is somewhat narrowed, terminates in the foot or basal disc. This secretes sticky mucus for adhesion on the sub strum.

2. The oral cone or hypostome

Around the base of which the tentacles are arranged, the mouth opens on top of this cone. When the animal is disturbed gently, it contracts rapidly, resulting in the shortening of the body.

Other features of Hydra

1. The enteron or gastro vascular cavity, which is only cavity in the body. It extends into the tentacles and has one inlet, the mouth, which at the same time acts as an exit.
2. The body wall is formed of an outer ectoderm and an inner endoderm, each of the layers being one cell in thickness. In between them there is a structure-less lamella, the Mesogleon.This construction continues into the tentacles.
3. The tentacles are thickly covered by protuberances, each carrying a battery of nematoblasts.
4. The buds are not always present and are of different sizes, they are usually connected to the body of the parent at the junction of its two lower thirds. At first, small, a bud then grows in size with an extension of the enteron, develops an oral cone and tentacles and finally separates from the parent to lead an independent life (asexual reproduction).
5. The gonads are merely accumulations of cells arising in definite sites on the body. Hydra is either hermaphrodite (monoecious), developing both types of gonads in the same individuals (but testes develop first and then are shed, ovaries developing afterwards-protandrous hermaphrodite) or unisexual (dioecious) i.e. the same sexes are separate. The testes appear as conical swellings near the oral cone, while the ovaries are formed near the middle of the body, and their size depends on their maturity.

Life cycle of Hydra

 The ectoderm

1. The myo- or musculo-epithelial cells have broad outer ends which meet to form the surface of the body (hence epithelial).They rest by their narrower inner ends on the mesoglea, where they give off processes or fibrils which run at right angles to the cells and parallel to the long axis of the body. These processes are contractile (hence muscular) and when they contract equally on all sides, the body is shortened.
2. The interstitial cells are small and rounded, and lie in the inter-stices between the musculo-epithelial cells. They are undifferentiated cells and seem to retain the properties of embryonic cells, as they may give rise to any of the other kinds of cells, especially the nematoblasts and germ cells.
3. The nematoblasts, peculiar to the coelenterates are extremely specialized cells for defense and offence. They appear in groups or batteries, particularly numerous on the tentacles. Different types of nematoblasts are known, the Iragest and most important of which are the penetrants. Each has a pear shaped body with the narrower end on the surface of the animal, and from it projects a small bristle called the cnidocil or trigger, which is thought to be sensory. Inside the body lies a pear-shaped capsule called the nematocyst with an operculum on the top.

The nematocyst is filled with fluid, and its outer end is tucked in and produced into a very long hollow capillary thread which lies coiled up in the capsule. The thread is thickened at its base where its carries a number of styles and many small barbs. Then the nematocyst is discharged by tactile or other stimuli, the thread is shot out and can penetrate the body of the prey or enemy into which a fluid is injected causing a paralyzing effect. The other nematocysts differ from the penetrants in the length of the thread and number of styles and barbs, as well as in the particular role that each plays in capturing the prey either by penetrating its tissues, adhering to it or coiling around its parts thus grappling it.

NB– Add a drop of acetic acid to a watch glass containing Hydra, the nematocysts will burst out, discharging the threads. Compare the threads in the different types of nematocysts.

4. The nerve cells are bi-polar and multi-polar cells which lie on the outer surface of the mesoglea and give off a number of branching processes (nerve fibers), which meet other processes of neighboring cells to form a nerve net. Another nerve net spreads over the inner surface of the mesoglea, in the endoderm.
5. The sensory cells are small columnar cells each having a small projection on the surface and connected at the base to a nerve fiber. Similar cells are found also in the endoderm.
6. The reproductive cells, only found in mature individuals, arise from interstitial cells by a process of cell proliferation, giving rise to the gonads which are either testes or ovaries.

 The Endoderm

1. The nutritive or musculo-nutritive cells are tall columnar cells which are drawn out of their bases i.e. towards the mesogloea, into contractile processes which run parallel to the circumference of the body, at right angles to its long axis. Their contraction causes a lengthening of the animal. The free ends of the cells form the lining of the enteron. Some of the cells carry flagella (producing a current of water), while others thrust out pseudopodia, and both contain food vacuoles in which some food particles are digested (intracellular digestion).
2. The glandular cells are wedged in between the nutritive cells. They are heavily granular and pour their digestive secretion into the enteron (Extra-cellular digestion).

Among the endodermal cells are found also sparse interstitial cells, which migrate from the ectoderm as needed and some sensory cells.

2. 2. Obellia, Order Calyptoblastea

• This is a marine coelenterate of the order Calyptoblastea common and cosmopolitan and appears in two forms, a hydroid and a medusoid.
• The hydroid form is colonial, sedentary and attached to seaweeds, shells and rocks in the intertidal zones but may be found at greater depths.
• The medusoid form is free swimming, is developed asexually from the hydroid form and itself reproduces sexually, giving rise to the hydroid form the two generations alternating one with the other.

1. The hydrocaulus or stem, on both sides of which stalked polyps or zooids arise in a cymase fashion.

• It is fixed to the substratum by a branching root-like portion called hydrorhiza.
• The stem is the form of tube, its cavity being the enteron which is continuous with that found in the different zooids through which the nutritive fluid is distributed.
• The wall of the tube is built on the same layers as those of zooids, namely the outer ectoderm, a thin mesogloea and an inner endoderm, all forming together the coenosarcs.
• The ectoderm secrets a horny flexible outer case, (exoskeleton), called the perisarc, which is continuous over the whole colony.
• The perisarc is annulated at intervals to allow bending.

2. The hydranth is the feeding zooid or polyp and is similar to hydra in many respects.

• It is protected by a cup-shaped structure, the hydrotheca continuous with the perisarc.
• The hydranth has a prominent oral cone or hypostome, in the center of which the mouth opens, and a ring of tentacles (about 24) project around its base.
• The body wall is built of ectoderm, mesogloea and endoderm, all continuous with those of the stem.
• The enteron does not project into the tentacles, the latter being solid (e.g. Hydra), having a solid central core of endoderm cells and carrying numerous nematoblasts on the surface.
• A circular shelf extends inwards from the hydrotheca at the hydranth base of the diaphragm, which narrows the opening into the stalk and prevents the passage of large food particles.

3. The blastostyle is the reproductive polyp, which has lost the tentacles, mouth and capacity to feed, and become specialized for asexual reproduction.

• Each one arises as hollow extension of the coenosarcs, which is enclosed in a flask-shaped extension of the perisarc called the gonotheca, with a distal aperture.
• A blastostyle grows at the base of the branch of the stem carry the hydranth. Along the sides of the blastostyle, medusac-buds are formed which on maturity separate off leaving the gonotheca through its aperture to swim freely away from the colony (thus disseminating the species).

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Life cycle of Obelia

The medusa of Obelia

1. The general form, umbrella-like (leptomedusa) with the rim fringed with tentacles carrying nematoblasts; with a dorsal convex surface, the exumbrella, and a ventral concave one, the sub-umbrella and with a manubrium hanging down from the centre of the sub-umbrella and bearing the mouth opening at its free end. A narrow ridge which is rich in muscle fibrils, the velum, projects inwards all-round the edge of the sub-umbrella.
2. The gastro vascular cavity (enteron).The mouth leads through the manubrium into a small gastric cavity in the centre of the umbrella. The cavity branches out into 4 radial canals and these join at their outer ends into a marginal or circular canal. All these canals are ciliated and carry the food particles from the manubrium in which digestion begins. The bases of the tentacles are swollen where digestive juices are also secreted.

The ectoderm covers the exumbrella and subumbrela surfaces, the tentacles and the manubrium, while the endoderm lines the system of the gastro vascular canals and the manubrium extends as a compact sheet between the canals and further as a solid core inside the tentacles. All the space left between these two epithelial layers is filled with thick mesogloea.

3. The statocysts are eight in number, two in each inter-radial region. They are tiny hollow balance organs which are lined with ectoderm, filled with fluid and contain minute calcareous grains.
4. The gonads are 4in number. The sexes are separate but not externally distinguishable. Gonads appear as ectodermal swellings, each hanging below a radial canal and discharge to the outside.
5. Physalia, Order Si­phonophora

The Portuguese man-of-war Physalia, is a complex colonial hydrozoan (Class Hydrozoa, Order Si­phonophora) exhibiting a high degree of polymorphism.

A single colony may consist of as many as 1,000 individ­uals and several types of polypoid and medusoid forms. The familiar iridescent colonies of Physalia are com­monly found along the beaches.

The most prominent feature of Physalia is a gas-filled float above, which is a sail-like crest. Physalia is transported by winds and oceanic currents, and its normal habitat is the open sea rather than the sandy beach where it is most often seen (and sometimes) by bathers. The sting from the nematocysts on the tentacles of Physalia can be painful when touched but is rarely dangerous, except to highly sensitive individuals.

Below the float are suspended numerous tentacles and other structures made up of several kinds of modified polyps and medusae. Thus, Physalia is an unusual cni­darian since a single colony contains both polypoid and medusoid individuals of several types closely joined to­gether, in contrast to separate polypoid and medusoid generations.

Physalia (Portuguese man-of-war)

1. Class Scyphozoa (Scyphomedusae)

This class includes all the larger jellyfishes, which are entirely marine animals in which the medusa is the predominant phase; the hydroid stage is very much reduced and includes in the life-history only as a polypoid larva. Gonads are endodermal and discharge into the enteron. The enteron is divided by ridges into four gastric pouches and bears some endodermal tentacles (gastric-filaments).There is an extensive system of radial canals.

Aurellia, Order Semaeostomeae.

This is a very common jellyfish of a world–wide distribution and belongs to the Order Semaeostomeae. It is readily recognized by its four pink horse–shoe shaped gonads which lie near the centre, embedded in the jelly.

The sexes are separate; fertilization occurs inside the female medusa. A planula larva is formed which after leaving the mother swims for some time and then settles down, transforming into a polyp known as the hydro tuba. This stores food, multiplies asexually and sooner or later develops a series of horizontal fissions which gradually deepen, forming a number of discs simulating a pile of saucers and hence called the scyhistoma. The process is known as strobilation. The discs separate successively giving rise each to an ephyra, which is a small larval medusa. This type of life-history is characteristic of the Scyphozoa.

Life cycle of Aurelia

The medusa

1. The general form, umbrella like but less convex than that of Obellia. A very short manubrium hangs from the sub-umbrella with four cornered mouth in its centre and the lips at the four
2. Corners are greatly elongated forming the oral lobes which are deeply grooved and bear nematoblasts along their edges.

2.The fringe of numerous short marginal tentacles are beset with stinging cells (nematoblasts) and are set closely together except where they are interrupted by eight marginal notches, each of which contains a tentaculocyst (sensory in function).

3. The gastro vascular system (enteron) starts by the mouth and leads into a small central gastric cavity that is extended into four gastric pouches (inter-radial) these contain the gonads and close inside each gonad ring is a prominent row of gastric filaments which carry nematoblasts that kill the prey swallowed alive. From each gastric pouch two unbranched adradial canals (eight in all) lead into marginal circular canal and a continuous current of water passes through them conveying food particles from the gastric pouches. From the circular a canal four branched interradial and four branched perradial canals lead inwards towards the gastric cavity and pouches passing the current of water out.
4. The gonads either testes or ovaries are horse–shoe-shaped, four in number in either case and lie in the floor of the gastric pouches. In the female the edges of the oral lobes are convoluted and white sports may be seen lodged in their convolutions. These are the developing planulae.

Stages in the development of Aurelia

1. The planula larva is ovoid and with a ciliated outer and an inner endoderm.
2. The hydra tuba resembles Hydra, it has a ring of long tentacles and a mouth but the gastric cavity is divided into four perradial gastric pouches.
3. The scyphistoma is a trumpet-shaped, with sixteen tentacles. Note the signs of horizontal fission on it.
4. The ephyra is a small medusa, the umbrella of which is divided into eight long forked arms. Note its manubrium with the mouth in the middle, its gastric cavity with gastric filaments and prolongations into the arms representing the branched per-and interradial canals and its eight prominent tentaculocysts.

1. Class Anthozoa (Actinozoa)

Sea anemone                                                                             Coral

This class comprises the sea anemones and reef building corals which are entirely marine. They are either solitary or colonial and exist only in the hydroid stage, the medusoid one being absent.

Internal anatomy of a sea anemone

The oral end is expanded radially into an oral disc bearing hollow tentacles. The enteron is subdivided by endodermal septa or mesenteries, and the entrance to the enteron is by a stomodaeum.The gonads are endodermal. The Actinozoan or Anthozoa are divided into two subclasses, the Alcyonaria with 6 orders and Zoantharia with 5 orders.

• Subclass Alcyonaria (Octocorallia)

Anthozoa have eight pinnate tentacles, eight mesenteries and one siphonoglyph in the stomodaeum.The muscle bands on the mesenteries face the siphonoglyph. They form a spicular skeleton in the mesogloea.

Order Alcyonacea

Genus Alcyonium

Alcyonium, the dead man’s fingers, is cosmopolitan and lives in colonies, growing on rocks and shells, the sexes are separate; the zygote forms a planula larva which settles down and becomes transformed into a polyp, which by budding gives rise to a colony.

The colony

A colony of Alcyonium

1. The tentacles are eight in number, pinnate and arranged in a circle around the oral disc, extensions of the enteron pass into them.
2. The mouth is slit like, found in the centre of the oral disc and leads into stomodaeum which hangs down in the enteron.
3. The mesenteries are eight in number and extend between the stomodaeum and the body wall, thus dividing the enteron into eight chambers. Below the stomodaeal level, the free ends of the mesenteries are thickened into convoluted cords or gastric filaments.

Classification

Two subclasses are commonly recognized: Alcyonaria (or Octocorrallia) and Zoantharia (or Hexacorallia). Within these, several orders are recognized.

• Subclass Octocorallia (=Alcyonaria)

Order Pennatulacea (sea pens, sea pansies)

Order Helioporacea (blue corals)

Order Alcyonacea (soft corals, sea fans, sea whips)

• Subclass Hexacorallia (=Zoantharia)

Order Actiniaria (sea anemones)

Order Scleractinia (true (stony or hard) corals

Order Corallimorpharia (mushroom (false) corals or called mushroom anemones)

Order Zoanthidea (zoanthids)

Order Antipatharia (black (thorny) corals)

Order Ceriantharia) (tube anemones)

Economic importance of Cnidaria

Anthozoans provide a number of values for human beings.

Coral reefs are major tourist attractions and also provide a habitat for fish, mollusks, urchins, and crustaceans that serve as food for people.

Anthozoans are used in the aquarium trade, to make coral jewelry, and scleractinian skeletons are even used as building materials and in bone grafts.

Despite these values, various human activities (fishing, development, marine pollution) have had negative effects on coral reefs, with more than half of the world’s coral reefs considered to be threatened.

PHYLUM CTENOPHORA

General characteristics

1) Biradial symmetry in most spp.

2) Ellipsoidal body shape

3) Mostly triploblastic

4) Only one sp. having cnidocytes w/ nematocysts; All spp. have coloblasts = adhesive cells.

6) Statocyst sense organs: controlling equilibrium.

7) No polymorphism.

8) Reproduction sexually in monoecious individuals (both male and female sex cells produced by some individuals.

9) Luminescence (biochemical light production process involving enzymatic hydrolysis of ATP).

Classification

Class Tentaculata – most representative type of phylum Ctenophora. Have tentacles, used mainly for food capture, (ciliated and usually w/out cnidocytes). Comb plates – ciliated structures used mostly swimming.

Class Nuda – lack tentacles.

Comparison with Cnidaria

• “Radial” symmetry
• True aboral-oral axis (polarity)
• Mesoglea in all spp.
• no true coelomic cavity
• Diffuse nervous “system”. Very simple and unspecialized.
• Lack true organ systems.

Contrast of Phyla Cnidaria and Ctenophora

1) Except one species of Ctenophorans, no stinging cells.

2) Mesoglea of Ctenophoras more specialized than Cnidarians (mesenchyme layer which can develop into true muscle tissue) – much more specialized than in Cnidarians- allows more efficient swimming.

3) Specialized structures, which include comb plates (ciliated and work in locomotion and food capture and colloblasts.

4) Mosaic pattern of development whereby collection of genetically distinct groups of cells are located throughout body.

5) Specialization esp. near oral/anal opening; pharynx connecting w/ oral/anal opening.

6) No polymorphism.

7) No colonial forms (free-living and living independently)

8) Simple anal openings at anterior end of organism for excretion (primarily of soluble wastes).

LECTURE FIVE: FLAT WORMS: PHYLUM PLATYHELMINTHES

General Features

Two major evolutionary advances in phylum

• Cephalization: Concentrating sense organs in the head region.
• Bilateral symmetry: Body can be divided along only 1 plane of symmetry to yield 2 mirror images of each other.
• First phylum with right and left sides.
• Acoelomates: Typical acoelomates have only one internal space, the digestive cavity.
• Without coelom (additional body cavity).
• Triploblastic: endoderm, ectoderm, and mesoderm. 1st phyla to have 3 germ layers.
• Protostomes: blastopore becomes the mouth. Incomplete gut with one opening.

Diagram of an Acoelomate Body Plan

Characteristics

• Commonly called They vary from a millimeter to many meters in length
• Some free-living while others are parasitic.

Classification

• Platyhelminthes is divided into three classes: Turbellaria (Planaria), Trematoda (flukes), and Cestoda (tapeworm)
• All members of Trematoda (flukes) and Cestoda (tapeworms) are parasitic

Class Turbellaria

• Mostly free-living forms. Most are bottom dwellers in marine or freshwater. Freshwater planarians are found in streams, pools, and hot springs while terrestrial flatworms are limited to moist places

Form and Function

• Epidermis and Muscles
• Most have cellular, ciliated epidermis on a basement membrane.
• Most turbellarians have dual-gland adhesive organs. Viscid gland cells fasten microvilli of anchor cells to substrate.
• Secretions of gland cells provide a quick chemical detachment.

Nutrition and Digestion

• Some have a mouth, pharynx, and intestine.
• In planarians, the Pharynx may extend through the ventral mouth
• The Intestine has three branches, one anterior and two posterior
• The Mouth of trematodes (parasitic flukes) opens near the anterior end.
• The Pharynx is not extensible
• While the Intestine ends blindly, varies in degree of branching

Planaria (Turbellaria)

• Carnivorous and detect food by chemoreceptors. Food trapped in mucous secretions from glands. Wrap themselves around prey. Extend the pharynx to suck up bits of food

Trematodes (parasitic flukes)

• Feed on host cells, cellular debris, and body fluids. Enzymes from the intestine are secreted for extracellular digestion. Phagocytic cells in gastrodermis complete digestion at intracellular level. Undigested food egested out the pharynx

Cestodes (tapeworm)

• Rely on the host’s digestive tract. Absorb digested nutrients

Excretion and Osmoregulation

• Flatworms have protonephridia (kidney) used for osmoregulation.
• Beating flagella drive fluids down collecting ducts. Wall of the duct beyond the flame cell bears folds or microvilli to resorb ions and molecules.
• Majority of metabolic wastes are removed by diffusion through body wall. Collecting ducts join and empty at nephridiopores

Nervous System

• Sub-epidermal nerve plexus resembles nerve net of cnidarians. One to five pairs of longitudinal nerve cords lie under the muscle layer.
• In freshwater planarians, brain is a bilobed cerebral ganglion (mass of nerve cells) anterior to the ventral nerve cords

Sense Organs

• Ocelli (light-sensitive eyespots) present in turbellarians, and larval trematodes

Tactile and chemoreceptive cells

• Abundant are statocysts for maintaining equilibrium and rheo-receptors which sense the direction of water currents.

Reproduction and Regeneration 

• Fission: Many turbellarians constrict behind the pharynx and separate into two animals in which each half regenerates the missing parts. This Provides for rapid population growth.
• Regeneration: If the head and tail are cut off, each end grows the missing part and retains polarity. Nearly all are monoecious (hermaphroditic) but cross-fertilize.
• Male Structures: One or more testes are connected to one vas deferens, which runs to a seminal vesicle. A nipple-like penis or extensible tentacle is the copulatory organ.
• Turbellarians develop male and female organs opening at a common pore. After copulation, eggs and yolk cells are enclosed in small cocoon which get attached by a stalk to plants. Embryos emerge and resemble little adults.

Classification of Phylum Platyhelminthes

Platyhelminthes are divided into three classes:

• Class Turbellaria – planaria
• Class Trematoda – flukes
• Class Cestoda – tapeworm

Class Turbellaria

• Mostly free-living. They Range from 5 mm to 50 cm long. Very small planaria swim by cilia.
• Others move by cilia, Glide over a slime track secreted by adhesive glands and also by rhythmical muscular waves pass backward from the head.

Class Trematoda

• All trematodes are parasitic flukes. Most adults are endoparasites (inside) of vertebrates. They resemble turbellaria but the tegument (skin) lacks cilia in adults. Sense organs are poorly developed.
• Adaptations for parasitism include: Penetration glands, Hooks and suckers for adhesion. They have a high reproductive capacity.

General Trematoda Life Cycle

Egg passes from definitive host and must reach water. Hatches into a free-swimming ciliated larva, the miracidium. Miracidium penetrates tissues of a snail. Transforms into a sporocyst. Sporocyst reproduces asexually to form redia. Rediae reproduce asexually and form cercaria. Cercariae emerge from the snail and penetrate a second intermediate host (fish). They develop into metacercariae (juvenile flukes). The Metacercaria develop into adults when eaten by definitive host.

Some serious parasites of humans and domestic animals are trematodes

Example: sheep liver fluke, human liver fluke

Sheep Liver Fluke (Fasciola hepatica)

Adult fluke lives in bile passageways in the liver of sheep. Eggs are passed out in feces. Miracidia hatch and penetrate snails to become sporocysts. After two generations of rediae, Cercaria encyst on vegetation and await being eaten by sheep. When eaten, metacercariae develop into young flukes.

Clonorchis sinensis: Human Liver Fluke

Most important human liver fluke, which is Common in China, Japan, and Southeast Asia. It also infects cats, dogs, and pigs. The Adult fluke is 10–20 mm long with an oral and ventral sucker.

Clonorchis Life Cycle (Liver Fluke)

Adults live in bile passageways of humans and other fish-eating mammals (sexual reproduction occurs here). Eggs containing a complete miracidium are shed into water with feces. The eggs hatch only when ingested by snails of specific genera. Miracidium enters snail tissue and transforms into a sporocyst Sporocyst produces one generation of rediae, which begin differentiation (asexual reproduction). Rediae pass into the snail liver, Turns into tadpole-like cercariae, which escape into water. The cercariae make contact with fish, bore into fish muscles or under scales, shed tail and turn into a metacercariae cyst. When a mammal eats raw fish, cyst dissolves and flukes migrate up bile duct. Heavy infection can destroy the liver and result in death

Control of parasites

Destroy snails and thoroughly cook fish

Schistosoma: Blood Flukes

• Over 200 million people infested with schistosomiasis. Blood flukes are common in Africa, South America, West Indies, the Middle and Far East.
• Sexes are always separate.
• There are 3 species with varied location: Large intestine, small intestine, urinary bladder

Schistosoma Life Cycle

• Eggs are discharged in human feces or urine. In water, eggs hatch as ciliated miracidia.
• The miracidia get in contact with a particular species of snail to survive.
• In the snail, they transform to sporocysts, which produce cercaria directly.
• The cercariae escape the snail and swim until they contact bare human skin or other host.
• Cercariae pierce the skin and shed their tails, enter blood vessels and migrate to the hepatic portal blood vessels (blood vessel from digestive tract to liver).
• They develop in the liver and migrate to target sites.
• The eggs are released by females, extruded through gut or bladder lining and exit with feces or urine. Eggs that remain behind become centers of inflammation.

Control:  proper disposal of human wastes

Schistosoma dermatitis (swimmer’s itch)

Occurs when cercariae penetrate an unsuitable host such as a human (our immune system fights them off leading to inflammation -itch). Normal host many be a bird or other animal.

Class Cestoda (tapeworms)

• Tapeworms have long flat bodies with scolex and a holdfast structure with suckers and hooks.
• Scolex is followed by a linear series of reproductive units or proglottids.
• They lack a digestive system.
• They lack sensory organs except for modified cilia.
• Tegument has no cilia.
• Entire surface of cestodes is covered with projections (microtriches) similar to microvilli seen in the vertebrate small intestine.
• Microtriches increase the surface area for food absorption.
• Nearly all cestodes require two hosts.
• Adult is parasitic in the digestive tract of the vertebrate.
• Over 1000 species of tapeworms known, infecting almost all vertebrates
• Most tapeworms do little harm to host

Taenia saginata: Beef Tapeworm                                                                    

• Lives as an adult in the digestive canals of humans.
• The juvenile form is found in intermuscular tissue of cattle.
• Mature adults can reach over 10 meters in length with over 2000 proglottids (segments containing reproductive organs).
• The Scolex has four suckers but no hooks.
• Gravid proglottids (with shelled, infective larvae) pass in feces, single. Proglottids rupture as they dry.
• Embryos are viable for five months and are picked up by grazing cattle.

Beef Tapeworm Life Cycle

• Cattle swallow shelled larvae that hatch as oncospheres, which use hooks to burrow through the intestinal wall into blood or lymph vessels.
• When they reach voluntary muscle, they encyst to become “bladder worms” (cyst that resembles a bladder)
• When the infected meat is eaten, the cyst wall dissolves and the scolex evaginates to attach to intestinal wall. New proglottids develop in 2–3 weeks. Infected individuals expel numerous proglottids daily.
• Infection can be avoided by eating only thoroughly cooked beef.

Other types of Tapeworm:

Taenia solium: Pork Tapeworm

Diphyllobothrium latum: Fish Tapeworm

Echinococcus granulosus: Unilocular Hydatid

LECTURE SIX: PHYLUM ROTIFERA (WHEEL ANIMALS)

• The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals.
• Most are around 0.1–0.5 mm long (although their size can range from 50 μm to over 2 mm), and are common in freshwater environments throughout the world with a few saltwater species.
• Some are free swimming and truly planktonic, others move by inch worming along a substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate.
• About 25 species are colonial, either sessile or planktonic.
• Most species of the rotifers are cosmopolitan, but there are also some endemic species.
• About 2200 species of rotifers have been described.

Anatomy

• Rotifers have bilateral symmetry and a variety of different shapes.
• The body of a rotifer is divided into a head, trunk, and foot, and is typically somewhat cylindrical.
• There is a well-developed cuticle, which may be thick and rigid, giving the animal a box-like shape, or flexible, giving the animal a worm-like shape; such rotifers are respectively called loricate and illoricate. Rigid cuticles are often composed of multiple plates, and may bear spines, ridges, or other ornamentation. Their cuticle is non-chitinous and is formed from sclerotized proteins.
• The most distinctive feature of rotifers is the presence of a ciliated structure, called the corona, on the head.
• The trunk forms the major part of the body, and encloses most of the internal organs.
• The foot projects from the rear of the trunk, and is usually much narrower, giving the appearance of a tail.
• The cuticle over the foot often forms rings, making it appear segmented, although the internal structure is uniform.
• Many rotifers can retract the foot partially or wholly into the trunk. The foot ends in from one to four toes, which, in sessile and crawling species, contain adhesive glands to attach the animal to the substratum.
• In many free-swimming species, the foot as a whole is reduced in size, and may even be absent.

Digestive system

• The coronal cilia create a current that sweeps food into the mouth. The mouth opens into a characteristic chewing pharynx (called the mastax), sometimes via a ciliated tube, and sometimes directly.
• The pharynx has a powerful muscular wall and contains tiny, calcified, jaw-like structures called trophi, which are the only fossilizable parts of a rotifer.
• The shape of the trophi varies between different species, depending partly on the nature of their diet.
• In suspension feeders, the trophi are covered in grinding ridges, while in more actively carnivorous species, they may be shaped like forceps to help bite into prey.
• In some ectoparasitic rotifers, the mastax is adapted to grip onto the host, although, in others, the foot performs this function instead.
• Behind the mastax lies an oesophagus, which opens into a stomach where most of the digestion and absorption occurs.
• The stomach opens into a short intestine that terminates in a cloaca on the posterior dorsal surface of the animal.
• Up to seven salivary glands are present in some species, emptying to the mouth in front of the oesophagus, while the stomach is associated with two gastric glands that produce digestive enzymes.
• A pair of protonephridia open into a bladder that drains into the cloaca. These organs expel water from the body, helping to maintain osmotic balance.
• Males do not usually have a functional digestive system, and are therefore short-lived, often being sexually fertile at birth.

Feeding

• Rotifers eat particulate organic detritus, dead bacteria, algae, and protozoans. They eat particles up to 10 micrometres in size.
• Like crustaceans, rotifers contribute to nutrient recycling. For this reason, they are used in fish tanks to help clean the water, to prevent clouds of waste matter.
• Rotifers affect the species composition of algae in ecosystems through their choice in grazing. Rotifers may be in competition with cladocera and copepods for planktonic food sources.

Nervous system

• Rotifers have a small brain, located just above the mastax, from which a number of nerves extend throughout the body.
• The number of nerves varies between species, although the nervous system usually has a simple layout.
• Close to the brain lies a retrocerebral organ, consisting of two glands either side of a medial sac.
• The sac drains into a duct that divides into two before opening through pores on the uppermost part of the head. Its function is unclear.
• Rotifers typically possess one or two pairs of short antennae and up to five eyes. The eyes are simple in structure, sometimes with just a single photoreceptor cell.
• In addition, the bristles of the corona are sensitive to touch, and there are also a pair of tiny sensory pits lined by cilia in the head region.

Reproduction and life cycle

• Rotifers are dioecious and reproduce sexually or parthenogenetically. They are sexually dimorphic, with the females always being larger than the males.
• In parthenogenetic species, males may be present only at certain times of the year, or absent altogether.
• The female reproductive system consists of one or two ovaries, each with a vitellarium gland that supplies the eggs with yolk.
• Each ovary and vitellarium form a single syncitial structure in the anterior part of the animal, opening through an oviduct into the cloaca.
• Males have a single testicle and sperm duct, associated with a pair of glandular structures referred to as prostates (unrelated to the vertebrate prostate).
• The sperm duct opens into a gonopore at the posterior end of the animal, which is usually modified to form a penis.
• The gonopore is homologous to the cloaca of females, but in most species has no connection to the vestigial digestive system, which lacks an anus.
• Fertilization is internal. The male either inserts his penis into the female’s cloaca or uses it to penetrate her skin, injecting the sperm into the body cavity.
• The egg secretes a shell, and is attached either to the substratum, nearby plants, or the female’s own body.
• A few species, such as Rotaria, are ovoviviparous, retaining the eggs inside their body until they hatch.
• Most species hatch as miniature versions of the adult. Sessile species, however, are born as free-swimming larvae, which closely resemble the adults of related free-swimming species.
• Females grow rapidly, reaching their adult size within a few days, while males typically do not grow in size at all.
• The life span of monogonont females varies from two days to about three weeks.

Recent transitions

• Loss of sexual reproduction can be inherited in a simple Mendelian fashion in the monogonont rotifer Brachionus calyciflorus: This species can normally switch between sexual and asexual reproduction (cyclical parthenogenesis), but occasionally gives rise to purely asexual lineages (obligate parthenogens). These lineages are unable to reproduce sexually due to being homozygous for a recessive allele.
• Males in the class Monogononta may be either present or absent depending on the species and environmental conditions.

Anhydrobiosis

• Bdelloid rotifer females cannot produce resting eggs, but many can survive prolonged periods of adverse conditions after desiccation.
• This facility is termed anhydrobiosis, and organisms with these capabilities are termed anhydrobionts.
• Under drought conditions, bdelloid rotifers contract into an inert form and lose almost all body water; when rehydrated they resume activity within a few hours.
• Bdelloids can survive the dry state for long periods, with the longest well-documented dormancy being nine years.
• While in other anhydrobionts, such as the brine shrimp, this desiccation tolerance is thought to be linked to the production of trehalose, a non-reducing disaccharide (sugar), bdelloids apparently cannot synthesise trehalose.

Predators

• Rotifers fall prey to many animals, such as copepods, fish (e.g. herring, salmon), bryozoa, comb jellies, jellyfish, and starfish.
• Rotifers are an important part of the freshwater zooplankton, being a major food source and with many species also contributing to the decomposition of soil organic matter.

LECTURE SEVEN: PHYLUM NEMATODA

Feeding

• Near mouth there are usually 16 hair-like sensory organs.
• The mouth is often equipped with piercing organs called stylets – sharp spikes used to kill cells or move through the dirt
• Food passes through the mouth as a result of the sucking action of a muscle chamber called the pharynx.
• Food then goes directly to the digestive tract, where it broken down and nutrients are absorbed.
• Digestive tract connects directly to the anus.
• Can live almost anywhere, species can be free-living or parasitic.

Respiration

• Breathe by simple diffusion.

Renette Cells

Circulation

• No circulatory system
• Foods circulate in pseudo-coelom through body movements

Excretion

• Have excretory ducts that permit them to conserve H₂0 and live on land
• Unique excretory system of collecting tubules or renette cells (excretory glands)
• Nitrogenous waste is excreted in the form of ammonia through the body wall
• Excretory system, if present, empties through an anterior, ventromedial porus.

Response

• Have a nervous system with pharyngeal nerve ring
• Unique cephalic sense organs, amphids
• Some with caudal sense organs, known as phasmids
• The muscles are activated by two nerves that run the length of the nematode on both the dorsal (back) and ventral (belly) side
• At the anterior end of the animal, the nerves branch from a dense circular nerve ring surrounding the pharynx, and serving as the brain.
• Smaller nerves run forward from the ring to supply the sensory organs of the head.
• The body of nematodes is covered in numerous sensory bristles and papillae that together provide a sense of touch.
• Behind the sensory bristles on the head lie two small pits, or amphids.

Movement

• Nematodes move by undulations or wave-like motions of the body.
• The muscles are able to “manipulate” each other to contract/relax accordingly
• There are the four “fields” of longitudinal muscles.
• Because the pseudocoelomic fluid is incompressible, the internal pressure increases causing stretching of muscle cells in another part of the body.
• Through this system of local contractions of the muscle fields the dorsal and ventral longitudinal musculature act as antagonists, producing sinusoidal waves along the length of the nematode’s body.
• Most nematodes lie on their sides and the resulting dorsi-ventral undulations move the nematode in the horizontal plane through an aquatic medium

Reproduction

• Basic male reproductive structures include:
• One Seminal vesicle – sperm accumulate here
• Vas deferens opening into a cloaca
• Bursae – thin cuticle extensions
• Spicules

Basic Female reproductive structures: One or two ovaries, seminal receptacles, Uteri, ovijector, vuvla

Reproduction

• All nematodes lay eggs
• Syngamy, or cross fertilization, is common in most nematodes.
• Hermaphroditism can occur
• Parthenogenesis is also a normal means of reproduction in some nematodes.
• Unique to animal kingdom, nematodes produce ameboid sperm – allows sperm to crawl to ova against hydrostatic pressure
• Internal fertilization
• Dioecious

Below are two lists of organisms and the Nematodes that infect them, this is only a minute example covering only two groups of organisms Commercial Plants and Domestic Animals, in the plant list many of the species listed for one plant group also infest other plant groups.

LECTURE EIGHT: PHYLUM ANNELIDA

• The Annelida comprises 15,000 or so species of segmented worms.
• They inhabit marine, freshwater, and moist terrestrial environments.
• They range in size from several mm up to 9 feet in length (but with a diameter only a little greater than that of a pencil).

Structure

• The dominant structural feature of annelids is their segmentation.
• Membranous septa (cross walls) separate the segments, with features like blood vessels and the gut passing through them.
• These septa permit muscular contraction and the response of the hydrostatic skeleton to be confined to one area, and allow much finer movement than was possible with the nematodes, which had no septa.
• Segmentation in an animal is also called metamerism. The segments are called metameres.

Nervous System and Movement

• Annelids have a large group of ganglia in their heads that some would call it a brain. A ventral nerve cord with a ganglion in each segment leads from the brain to the rear of the worm.
• The movement of the annelids depends on the interaction of muscles with a hydrostatic skeleton.
• Earthworms have both circular and longitudinal muscles. When the circular muscles in the front of the worm contract against the fluid in the coelom, the front elongates and gets thinner, and the worm extends forward.
• Then the worm drives its front setae into the soil and contracts its longitudinal muscles. This drags the rear of the worm forward.
• Control of the crawling sequence is sufficiently decentralized so that even a headless worm can still crawl. Annelids move in other ways, too. Some polychaetes can swim, and some are pelagic (spending their whole lives swimming or floating in the water column of the open sea).
• Many leeches can “walk” with a hunching, looping head/tail/ head/tail type of motion, and they can also swim.

Feeding

• Three types of feeding occur among the annelids– predation, deposit feeding and filter feeding.
• All classes of annelids contain some members that are predatory (they hunt down living organisms and eat them).
• Leeches are exclusively predatory.
• Many polychaetes and most earthworms are deposit feeders.
• They ingest sediment or soil, extract the nutrients, and pass the waste out through the anus. An earthworm can eat its own weight in soil and decaying vegetation each day.
• Filter-feeding polychaetes use feathery tentacles to filter small particles from the water.
• When the longitudinal muscles contract, the worm shortens. When the setae extend, the worm is anchored.
• Contraction of the circular muscles causes the worm to get thinner. Because the fluid in the coelom is incompressible, a thinner worm must become a longer worm.

Gas Exchange and Excretion

• Annelids exchange gases across the integument or outer body surface. Marine and freshwater species often have extensions of the body that increase surface area and enhance gas exchange. These extensions may be tentacles or parapodia.
• Terrestrial species, in general, have an advantage in gas exchange since air contains about 21% oxygen as opposed to water which contains only from 0% to 1% oxygen. All gas exchange requires a moist surface over which the gases can diffuse.
• In land species, this moisture is provided by mucus, body fluids, or wetness in the environment.
• The polychaetes have highly vascularized, paddle-like parapodia in each segment.
• The parapodia also act as gills because they increase the surface area through which oxygen can diffuse. A closeup of a parapodium appears below.
• Earthworms also exchange gases through their body wall without special gas exchange structures, but they have a complicated circulatory system too.
• In annelids, a pair of highly coiled metanephridia in almost every segment removes nitrogenous wastes from the blood and coelomic fluid and dumps it to the outside.
• The metanephridium resorbs water and other valuable molecules before dumping the wastes.

Reproduction

• The earthworms have some asexual reproductive abilities, but most of their reproduction is sexual.
• All earthworms are hermaphroditic or monoecious (having both male and female organs in one individual). The alternative, dioecious, means having separate males and females.
• Oligochaete fertilization is internal, and the two worms press their bodies together and fertilize each other. Polychaetes have amazing powers of asexual regeneration.
• Sexually, the Polychaeta has both dioecious and monoecious members. Fertilization may be internal or external.
• Some polychaetes use a spectacular reproductive technique called swarming. The worms, normally quiet bottom dwellers, develop specialized segments (called epitokes) that are bursting with eggs and sperm. Then, at some cue (perhaps related to the moon), the epitokes break off and swim to the surface in vast numbers, where they shed their eggs and sperm in patches that may cover acres. Larvae formed by fertilization later sink to the bottom.
• The leeches reproduce exclusively by sexual means. All leeches are monoecious. They also do not add segments as they age.

Diversity

• The three classes of annelids are the: Polychaeta marine annelids, Oligochaeta includes earthworms and Hirudinea leeches

1. Class Polychaeta

• Almost all of the 10,000 species of polychaetes live in marine environments.
• They seem to be the most primitive class of annelids, but they show some highly specialized adaptations.
• Lifestyles include very active predators (with formidable, hardened jaws), burrowing deposit feeders, filter feeders, burrow dwellers, tube dwellers, and a few symbionts.
• Polychaetes have a wide variety of body forms, and may have many appendages: tentacles, setae, palps and parapodia.
• Polychaetes form a large percentage of the mud-dwelling fauna of the ocean floor.
• Their populations seem to be limited by predation, because when predators are excluded by cages, polychaete populations increase markedly.

1. Class Oligochaeta

• The 3,100 species of oligochaetes include two main groups: the earthworms and freshwater oligochaetes. Freshwater oligochaetes are smaller than earthworms, tend to be transparent, and have more prominent setae. There are also a few marine oligochaetes.
• Earthworms are important members of terrestrial ecosystems, where they serve as detritivores– heterotrophs that feed on partially decomposed particles of organic matter. In this role, they aerate the soil, improve soil texture, and make nutrients available to other organisms by bringing subsoil to the surface.

1. Class Hirudinea

• Real leeches range from 5 mm to 250 mm in length. Many leeches swallow small animals or kill them and suck out their juices. 75% of leeches are parasites that feed on the body fluids of a wide range of vertebrate and invertebrate hosts.
• Adaptations for this lifestyle include suckers, sharp, blade-like jaws, and chemical anesthetics, vasodilators, and anticoagulants.
• A bloodsucking leech has a highly distensible gut. It can take on 5-10 times its own weight in blood at one feeding, and then take up to a year to digest the meal.
• Wet tropical forests support a high density of terrestrial leeches that sense warm-blooded prey by their body heat.
• Leeches seem to be the most advanced, specialized, and highly modified annelids.
• Much of their internal segmentation has been lost, and their external rings (called annuli) do not correspond to the old annelid segments and do not increase in number with age. The different regions of the leech body are more specialized.
• Leech behavior tends to be more complex than that of other annelids.
• Some parasitic leeches serve as vectors and intermediate hosts for protozoan, nematode and flatworm parasites.
• On the other hand, leeches are also used to maintain circulation in limb and digit reattachment surgery.

LECTURE NINE: PHYLUM MOLLUSCA

• The Mollusca are eucoelomate, protostomes, lophophorates, bilaterally symmetrical, cephalized, and have a complete gut. They are not segmented.
• There are 80,000 or more living species of mollusks, and because of their shells, they also probably have the best fossil record of any group (about 35,000 species).
• Mollusks range in size from microscopic bivalves to giant squids 60 feet or more in length. They inhabit all marine and many freshwater and terrestrial environments.
• There are eight classes of mollusks, but we will cover four:
• Polyplacophora chitons
• Gastropoda snails and slugs
• Bivalvia clams, mussels, etc.
• Cephalopoda squids, octopuses

Structure

• The basic molluscan body plan (and perhaps ancestral body plan) is like a sandwich. The bulk of the animal, including the head and all the internal organs, is the visceral mass, and is the middle of the sandwich.
• Under the visceral mass is a muscular foot, and over the visceral mass is a heavy sheet of tissue called the mantle. The mantle may secrete a calcium carbonate shell on its top side.
• At the rear of the animal, the mantle extends beyond the visceral mass and creates a cavity called the mantle cavity. The mantle cavity contains gills known as ctenidia.
• In snails, a process called torsion has twisted the visceral mass so that the anus is near the mouth. In slugs, squid and octopi, the shell has disappeared or become reduced. In clams, the shell has become two hinged pieces that drape down on either side of the animal, and the foot extends from between the shells.

Locomotion

• Snails and chitons crawl using ripples of muscular contraction in their foot.
• Bivalves are more sedentary, and use the foot to burrow into sediments. A few bivalves, like scallops, can flutter their shells and actually swim for a short distance.
• Octopuses crawl on the sea floor, and squid swim in the open water by taking water into their mantle cavity and then expelling it through a short duct called a siphon.

Feeding

• There are two basic types of feeding that occur among the species of mollusks – predation (both herbivorous and carnivorous) and filter feeding.
• A scraping organ called the radula may be used to scrape up algae, to bore into the shells of other mollusks, or to deliver poison, as in the cone shells.
• Many aquatic mollusks, the bivalves in particular, are filter feeders, using a water current across the gills to simultaneously trap food particles and exchange gases.
• A few marine mollusks seem able to meet their nutritional requirements by direct uptake of dissolved organic materials from the environment.
• One mollusk with very objectionable feeding methods is the shipworm. This small bivalve bores into wood with its shell and feeds on the sawdust it excavates from its burrow. It is one of the few animals that can digest cellulose. Some shipworm tunnels can be 2 m long, and a piece of wood can be riddled with tunnels in a few months.

Gaseous exchange

• Most mollusks have true gills or ctenidia, and the bivalves have extensive gills that cover most of their body and are also a filter-feeding organ.
• Ctenidial cilia carry water over the gills in a direction opposite to that of the flow of blood in the gill tissue. This countercurrent exchange maximizes the diffusion gradients of oxygen and carbon dioxide between the water and the blood and ensures efficient gas exchange.
• Some species lack gills and rely instead on other gill-like structures (cerata) or on gas exchange across the body surface or mantle.
• Terrestrial gastropods generally lack gills and exchange gases across a specialized region of the mantle cavity.
• Air is moved in and out of a special opening into a highly vascularized cavity by action of the mantle. This structure functions as a lung, and the snails that have it are called pulmonates.

Circulation

• Most mollusks have an “open” circulatory system. Blood leaves the heart in a major vessel, but then runs freely over the organs and is collected in a series of tissue spaces called a hemocoel, and then is sucked up by the heart.
• Open circulatory systems are not very good at oxygenating tissues, they are sufficient for sluggish mollusks like snails and clams.
• Cephalopods, with their highly active lifestyles, have a closed circulatory system like the Annelida.
• Mollusks also have excretory organs called metanephridia, but in the mollusks they are also called kidneys. These take in coelomic fluid and remove the wastes from it while retaining valuable materials like water, sugars and salts.

Reproduction

• Reproduction in the mollusks is strictly sexual, but individuals may be monoecious or dioecious and fertilization may be either internal or external.
• Chitons and bivalves usually have separate sexes and fertilization is external. Sperm and eggs pass through ducts that carry gametes directly into the water.
• Gastropods and cephalopods also usually have separate sexes, but in these classes, fertilization is usually internal. These two classes are also notable for their courtship behaviors.

Diversity

The four most important classes of mollusks include the following.

• Polyplacophora the chitons
• Gastropoda snails and slugs
• Bivalvia clams, etc.
• Cephalopoda squids, octopuses

Polyplacophora.

• There are about 600 species of chitons.
• The chitons are flattened, elongated mollusks with a broad foot and eight shell plates. They are primarily herbivores in rocky intertidal zones, grazing on marine algae with a rasping action of the radula.
• These mollusks lack eyes, tentacles, and concentrated bundles of nerve cells in the head.
• When a chiton is disturbed or exposed by the receding tide the muscles in its foot contract like a suction cup, making it very difficult to detach. When dislodged, a chiton can roll itself up in a ball.

Gastropoda

• Gastropods have about 70,000 living species, about 80% of all mollusks.
• They are they only mollusks to have successfully invaded the land. One of the most interesting features of this group is a developmental process called “torsion”. After torsion, the body is twisted around in such a way that the mantle cavity is above the head (handy for withdrawal in times of danger).
• Torsion also brings the gills, anus, and kidney openings above the head. Cilia create currents that direct waste products away from the mouth and gills.
• Most gastropods crawl on their foot, searching for patches of algae or other food, and then rasp the food up with their radula.
• The slugs (on land) and the nudibranchs (in the ocean) have lost their shell. Some nudibranchs replace the protection offered by a shell with the protection offered by nematocysts.
• They eat cnidarians, and move the undischarged nematocysts to their top side. Other nudibranchs have skin glands that secrete sulfuric acid.

Bivalvia

• The bivalves have about 8,000 living species. They range in size from 1 mm to the Pacific giant clams, which can be three feet across, weigh 500 pounds, and live to be 100 years old.
• The main distinguishing feature of this class are the two shells hinged together by ligament and muscle. The head is not highly developed, lacking tentacles, eyes and radula. Bivalves use their foot for burrowing and anchoring.
• Bivalves are primarily filter feeders and live on algae and other particles in the water. They use extensions of the mantle to form siphons, and these move water in and out of the body.
• Typically, the only visible part of a bivalve is the rear end of the shell and the siphons extending above the sediment surface. If the animal senses movement nearby, it will withdraw the siphons, snap the shell shut, and perhaps burrow deeper into the sediments.
• Not all bivalves burrow in soft sediments. Mussels attach to hard substrates like wharf pilings with very strong threads, and oysters attach their shells to any hard substrate, including each other’s shells. Over time, they can create a massive aggregation of oysters called an oyster reef.

Cephalopoda

• There are about 650 species in the Cephalopoda, including the octopuses, squids, nautiluses and cuttlefish.
• Some cephalopods are only a few inches long, but the giant squid (60 feet or more) is the largest invertebrate.
• Except for the nautiluses, cephalopod shells are either internal or absent.
• Despite the mollusk reputation for slowness or even immobility, cephalopods are highly active predators. They have the ability to expel water through the exhalant siphon with great force and propel themselves through the water.
• Cephalopods have large complex eyes and many have long tentacles, some with suckers.
• Squids swim in the open water. Kills prey by biting it with hardened jaws injecting poison into it.
• Octopuses and squids have highly developed nervous systems. Octopuses are the most intelligent of all the invertebrates.

Human Impact and Evolution

• Used as food
• Their shells used for decoration, money, music, tools and utensils.
• The giant axon from the squid in understanding of how nerves function.
• They act as intermediate hosts of disease causing trematodes and therefore as vectors.

LECTURE TEN: PHYLUM: ECHINODERMATA

• Means “prickly skin” 6225 living species; >20,000 fossil species 15 classes of extinct species
• All marine; found in all oceans at all depths some of the most abundant of all marine invertebrates
• Unable to osmoregulate; even rare in brackish waters almost all are bottom dwellers a few are pelagic swimmers a few are commensal.
• In general they are not often prey to other species mostly drab colors but a few are red, orange, purple, blue, etc.

Distinctive Characteristics of Phylum:

1. Most with pentamerous (=pentaradial) radial symmetry
2. No distinct head or brain (no cephalization)
3. Most have endoskeleton of calcium plates
4. Unique water vascular system for feeding and movement
5. Dermal branchiae for gas exchange
6. No real circulatory system
7. No excretory system
8. Sense organs poorly developed
9. Pedicellariae for protection

Body Wall

Epidermis

• Outer surface covered by epidermis made up of epithelial cells, ciliated mucous cells, ciliated sensory cells and nerve plexus in basal part of epidermis.

Dermis

• Below epidermis is thick dermis made of connective tissues with Lots of collagen fibers and secretes skeletal pieces = ossicles =
• Ossicles are bony plates made of calcium Crystals. Each ossicle represents a single crystal of magnesium rich calcite (6(Ca.Mg)C0₃) formed within cells of dermis. In many classes ossicles have bony projections for defense.
• Echinoderms can vary rigidity of dermis. Pliability of collagen fibers is under nervous control = “catch collagen”. Soft and pliable rigidity allows the animal to hold various postures for long periods without muscular effort.
• Beneath dermis is layer of outer circular and inner longitudinal muscles. The animals have a true coelom lined with

Movement

• Movement & food gathering done predominantly by water vascular system.
• There is a second, separate coelomic compartment unique to echinoderms derived from coelom and lined with ciliated epithelium. The whole system operates hydraulically filled with fluid (mainly sea water and some proteins and cells.
• Internal canals connect to the outside through the madreporite leads to stone canal (contains calcareous deposits) which joins ring canal just inside and around the mouth.
• Long radial canals extend into each arm. In arm, lateral canals branch off radial canals have valves to prevent backflow and Lead to small muscular sacs that serve as fluid reservoirs = ampullae connected to muscular tube feet.
• Tube feet are concentrated in ambulacral groove. The tip of the tube feet are flattened, forming suction like cups that can produce strong force
• Tube feet are used to cling to substrates, move and to feed.
• Most echinoderms don’t have large muscles. Muscles mainly used to move tube feet but some also attached to ossicles to allow them to bend and flex.
• Water vascular system also compensates for the absence of a blood circulatory system

Feeding & Digestion

• Echinoderms are particle feeders, scavengers or predators. There are no parasitic species. They have simple and usually complete digestive tract but functional anus is often reduced.
• The stomach has 2 chambers: cardiac & Digestive enzymes are secreted into stomach by pyloric caecae.

Respiration

• Tiny saclike projections which extend through epidermis = dermal branchae (or papulae) are used in the exchange respiratory gasses. They also get rid of ammonia (N-wastes).
• The same functions are also shared by tube feet in most groups.

Circulation

• Echinoderms rely mainly on coelomic circulation for transport of gasses and nutrients. Ciliated lining circulates fluids around body cavity and into dermal branchiae. Coelomic fluid contains amoeboid cells.
• They do have a blood vascular system (= hemal system) with heart but it’s usually rudimentary and its function unclear. May play some role in distributing nutrients.

Nervous System

• Echinoderms have no brain or centralized processing area. Circumoral ring and radial nerves branching from it helps coordinate movement of arms and movement of the starfish in general.
• Tube feet are innervated by nervous system which enables all feet to move in a single direction. Echinoderms have few specialized sense organs. They have some simple tactile, chemical and photoreceptors and

Protection

• In many starfish the body surface bears small jaw-like Some are stalked while others are sessile (unstalked). The pedicellariae protect against animals and debris that settle on the animals’ surface.

Excretion

• Removal of nitrogen wastes (mainly ammonia) is through the body surface, dermal branchiae and tube feet
• Some amoeboid cells can also engulf nitrogen wastes and move them to the outside through the dermal branchiae or tube feet

Reproduction & Development

• Sexes typically separate = dioecious. Fertilization is external. They produce characteristic ciliated, free-swimming, planktonic larva = bipinnaria which are bilaterally symmetrical.
• Undergoes metamorphosis to become radially symmetrical adult. Early developmental stages are similar in all classes. Some can also reproduce asexually by fragmentation.
• Many also have excellent powers of regeneration and can regenerate from 1/5th of oral disc & a single arm but may require up to a year. Some deliberately cast of an arm as a means of asexual reproduction. Don’t seem to age and can live forever.

Ecology

• A wide variety of other animals make their homes in or on echinoderms, including: algae, protozoa, ctenophores, turbellaria, barnacles, copepods, decapods, snails, clams, polychaetes, fish and other echinoderms

Echinoderm Classification

The phylum consists of five classes: Asteroidea (Starfish, sea stars, sea daisies), Ophiuroidea (Brittle stars, basket stars, and serpent stars), Echinoidea (sea urchins, heart urchins, sand dollars & sea biscuits) Holothuroidea (Sea Cucumbers) and Crinoidea (sea lilies, feather stars).

Class Asteroidea

• Single ambulacral groove
• Have a large coelom where all the main organs occur
• Specialized pinchers found on the aboral surface
• Can reproduce asexually by disk division
• Sexual ReproductionDioecious with sperm or eggs produced in 2 or more gonads in each arm
• Larval stage = bipinnaria
• Many species autotomize, leaving predators with a nutritious souvenir while they escape
• Most spp. can regenerate from fragments that include the disk

Class Ophiuroidea

Defining Characteristics

• Well-developed ossicles in the arms forming a system of articulating vertebrae
• The oral surface bears 5 pair of bursal sacs

Reproduction

• Similar to Asteroids; yet a pluteuslarva is formed
• Regenerate well, and one spp., in our area reproduces asexually by disk division

Class Echinoidea

Defining characteristics

• Ossicles are joined to form a rigid test
• Adults possess a feeding structure called Aristotle’s lantern
• Two attributes: mobile spines, and hollow skeleton or test

Pedicellariae

• Pedicellariae prevent fouling of test and are used in defense
• More complex than sea stars and are located on tall moveable stalks

Reproduction

• Most conspicuous organs are those responsible for reproduction
• At spawning the entire coelom will fill with sperm or eggs

Pluteuslarva is formed

Class Holothuroidea

Defining characteristics

• Worm shaped body, greatly elongated along the aboral and oral axis
• The calcareous ossicles are reduced in size and embedded individually in the body wall
• Highly branched muscular respiratory structures

Holothuroidea Feeding

• Possess retractile feeding tentacle that surrounds the mouth
• While suspension or deposit feeding each tentacle is cleaned in the mouth

Holothuroidea Structure

Defense

• Many spp. have powerful toxins in the body wall
• Cuverian tubules Sticky and toxic tentacles which are shot out the anus
• Also eviscerates to avoid predation. Internal organs regenerate after a period of time

Economic/Human Impacts

• Feed on coral polyps, and sometimes attack in “herds”, the number of reef attacks is increasing, sometimes results in extensive damage, very expensive to control outbreaks.
• Sea urchins destroy kelp forest, but are preyed on by sea otters.
• Predatory starfish can devastate commercial clam or oyster beds e.g. a single starfish can eat a dozen clams or oysters in a day, sometimes an infestation is treated with quicklime destroys dermal branchiae and kills animal
• Used as food: e.g. in China and Pacific Islands sea cucumbers are boiled and dried and eaten as a delicacy or used as a food flavoring in some areas collecting has severely depleted their populations e.g. roe (gonads & eggs) are sold, raw or roasted, as a delicacy in Japan and in sushi restaurants >30M pounds of urchins were harvested in 1986
• Used in developmental research.

LECTURE ELEVEN: THE PHYLUM ARTHROPODA

The arthropods, largest phylum of animals in the world. Includes: spiders, mites, scorpions, ticks, crustaceans, millipedes, centipedes and insects.

Characteristics

1. Bilateral symmetry, metamerism, some somites (segments) fused to form tagmata
2. Three body regions: head, thorax, and abdomen
3. Appendages jointed and often specialized
4. Exoskeleton (cuticle) made chiefly of chitin; some proteins and lipids also
5. Muscular system complex, no cilia
6. Coelom reduced and filled with blood to form hemocoel
7. Complex digestive system
8. Open circulatory system
9. Respiration by gills, trachea, or book-lungs
10. Excretory system of Malpighian tubules in some; coxal, maxillary or antennal glands in others
11. Nervous system of annelid plan with highly developed sensory organs
12. Sexes usually separate, metamorphosis in some, internal fertilization, growth with ecdysis (molting)

Reasons for Success

1. A versatile exoskeleton
2. Segmentation and appendages for more efficient locomotion
3. Air piped directly to tissues
4. Highly developed sensory organs
5. Complex behavior
6. Reduced competition for resources through metamorphosis

Metamorphosis

A change in body plan:

• Direct development: example humans
• Indirect development: the larval or juvenile stage does not resemble the adult
• Ecdysis

The molting of the cuticle to accommodate growth, generally occurs between developmental stages

Exoskeleton

Cuticle (outer covering) secreted by epidermis, layered

• Epicuticle: outer, thin layer of protein and lipids
• Procuticle: inner, thicker layer of chitin and protein
• exocuticle
• endocuticle
• Tanning
• Laminated
• Ecdysis

Subphylum Trilobita: trilobites

• Once numerous, now extinct

Subphylum Chelicerata: spiders, ticks, mites, scorpions, horseshoe crabs

• Six pairs of appendages
• Pair of chelicerae
• Pair of pedipalps
• Four pairs of walking legs
• No mandibles
• No antennae

Class Merostomata: horseshoe crabs

• Unsegmented large carapace
• Broad abdomen
• Telson (spine-like tail)
• Book gills

Class Arachnida: spiders, ticks, mites and scorpions

Order Araneae: spiders

• Tagmata of cephalothorax and abdomen
• Tagmata joined by pedicel
• Chelicerae function as fangs and deliver poison
• Breath by book lungs or trachea, with spiracles
• Unique excretory system of malpighian tubules
• Coxal glands (modified nephridia)
• Eight simple eyes
• Sensory setae (bristles) on body
• Silk glands and spinnerets

Order Scorpionida: scorpions

• Short cephalothorax and long segmented abdomen
• Abdomen divided into preabdomen (broad base) and postabdomen (tail-like with stinger)

Order Opiliones: harvestmen (daddy longlegs)

Order Acari: ticks and mites

Subphylum Crustacea: crayfish, lobsters, shrimp, crabs

1. Nearly all aquatic
2. Two pair antennae
3. Biramous appendages
4. protopodite = basal segment Y shaped

• endopodite = medial ramus
• exopodite = lateral ramus

Class Malacostraca: lobsters, crayfish, shrimp, krill, crabs, mysids, isopods, amphipods

Order Isopoda: “pillbugs”

Order Amphipoda

Order Decopoda: crabs, lobster, crayfish, shrimp

1. Largest
2. Typical ex. crayfish

Body Structure

• cephalothorax and abdomen
• carapace
• paired appendages
• antennae, mandibles, maxillae, maxillipeds, walking legs, swimmerets
• telson and uropods

3. 5 pairs of walking legs
4. Chelipeds
5. Herbivorous, carnivorous, scavenger
6. Serially homologous
7. Compound eyes

Class Branchiopoda: water fleas

1. Freshwater
2. Flattened

Class Cirripedia: barnacles

1. Sessile
2. Some parasitic

Subphylum Uniramia: insects and myriapods

1. Five classes
2. Over million species

Class Diplopoda: millipedes

Class Chilopoda: centipedes

Classes Pauropoda and Symphyla

Class Hexapoda: insects

1. Body Structure

• 3 body regions: head, thorax, abdomen
• Single pair antennae
• Compound eyes
• One or two pairs of wings
• 3 pairs walking legs
• Thorax has 3 fused segments

1) Prothorax

2) Mesothorax

3) Metathorax

• Insect flight

1) Synchronous flight

2) Asynchronous flight

• Nutrition

1) Labium

2) Proboscis (modified maxilla)

• Circulation and temperature

1) Open circulatory system

2) Ectothermic

• Nervous system

1) Odor

2) Tactile (setae)

3) Johnston’s organs (statocysts)

4) Tympanic organs (hearing)

5) Compound eye and ommatidia

• Excretion: malpighian tubules
• Chemical regulation: pheromones
• Reproduction and development

1) Metamorphosis = change

2) Larval instars = immature forms

3) Types of metamorphosis:

• a-metabolous = only change in body size and maturity
• pauro-metabolous = many molts, gradual change from larva into adult form (eggs>nymph>adult)
• hemi-metabolous = immatures are aquatic (naiads), adults are not
• holo-metabolous = immatures most different from adult, last stage before adult called pupa (eggs>larva>larva>pupa>adult)

4) Cocoon, chrysalis, puparium

• Insect behavior and social insects

The Origin and Evolution of Arthropods

Arthropods appeared on earth about 550 million years ago. They are believed to have evolved from annelids basing on the following similarities:

1. Metamerism with tendency for segments to become specialized.
2. Similar nervous system with paired ganglia in each segment.
3. Some have same type of excretory system.
4. Spiral cleavage in primitive members

Soft cuticle of a segmented worm was hardened by deposits of additional proteins and calcium.  The hard sections of cuticle were still separated from each other by flexible sutures and joints. This has provided protection from predators & environmental hazards and a more secure site for attachment of muscles. Parts of hard exoskeleton became pivots and levers for jointed appendages.

Timelines for the Evolution of insects

Ordovician Period (480 million years ago). The class of insects appears on Earth, at about the same time terrestrial plants appeared.

Devonian Period (about 400 million years ago). They become the first animals to develop flight.

Carboniferous Period (356 to 299 million years ago). The Pterygotes (winged insects) undergo a major radiation.

Permian Period (299 to 252 million years ago). The Endopterygota (insects that go through different life stages with metamorphosis) undergo another major radiation. Most extant orders of insects develop.

Permo-Triassic boundary. Many of the early groups became extinct during the mass extinction. The largest extinction event in the history of the Earth, around 252 million years ago.

Triassic period (252 to 201 million years ago). The survivors of the mass extinction evolved to what are essentially the modern insect orders that persist to this day.

Jurassic period (201 to 145 million years ago). Appearance of the most modern insect families.

Cretaceous period (145 to 66 million years ago). In an important example of co-evolution, a number of highly successful insect groups — especially the Hymenoptera (wasps, bees and ants) and Lepidoptera (butterflies) as well as many types of Diptera (flies) and Coleoptera (beetles) — evolved in conjunction with flowering plants

Cenozoic period (about 65 million years ago). Many modern insect genera developed. Insects from this period onwards frequently became preserved in amber, often in perfect condition.  Such specimens are easily compared with modern species, and most of them are members of extant genera.

Factors Leading To the Success of Insects

Insects are the most successful in the animal kingdom, being able to survive in all inhabitable environments. This is because they have developed morphological, physiological and developmental adaptations which are not common in other animal groups.

Water loss reduction

• Insects have several mechanisms to reduce water loss. One structural mechanism is the waxy coating over the exoskeleton of insects.
• In addition, most insects do not excrete liquid water; they reabsorb water from their waste products.

Diversity

• Because of their great diversity, insects provide an understanding of the adaptability of animal systems and biological mechanisms that survive the physical and biological challenges necessary to exist in these environments
• Why have insects been so successful? Several ideas, most of which are interrelated, have been proposed to explain the success of insects.
• Great diversity and abundance is based on their high reproductive capacity. Most insects produce huge numbers of offspring and many species produce several generations each year. This large reproductive capacity is related to insects’ adaptability to a wide range of environmental factors.

Small Size

• A second factor relates to their small size. There are several advantages to being small:
• Individuals require less energy and time to complete development
• Insects easily can hide from predators by using microhabitats
• Again because of the use of microhabitats, more habitats are available for use by insects compared to larger animals
• The muscle system of insects is similar to vertebrates, but due to their small size, muscular action is more efficient
• Solar radiation is used by insects to warm their bodies
• Because of their small size, insects can be moved by the wind. This movement may be over long distances or short distance random movements.

Special Appendages

• Insects have different types of appendages (for example, legs, wings, and mouthparts).
• Insect species possess various forms of mouthparts that are used to feed on a wide variety of substances.
• Most plants and animals are fed upon by at least one species of insect. Many serve as hosts to a wide variety of insect species.

Legs

• Insects have diverse leg types which function in a variety of ways. For example, grasshoppers and fleas have legs adapted for jumping.
• Many aquatic insects have legs which are modified for swimming.
• Many species of predaceous insects have modified front legs which are used to catch and hold prey.

Wings

• Many insect species have wings, which increase their ability to disperse within habitats and between habitats.
• Insects are the only invertebrate animals which have wings and these structures have been advantageous for insect survival.
• An additional adaptation found in certain types of insects is the ability to fold up the wings and protect these delicate structures under wing covers.

Development

• Another reason for the success and diversity of insects is related to the type of development shown by most insect species.
• As an insect grows it goes through life stages that may be very different in structure and function. Many insect species are found in one type of habitat in the immature stages and another habitat in the adult stage. For example, a caterpillar feeds on plant matter while an adult butterfly feeds on nectar.
• For many insect species, the immatures and adults feed on different foods and do not compete with each other for food: clearly an advantageous adaptation.

Diverse Development

• One additional factor contributing to the success of insects is the tremendous variety of developmental patterns.
• Insects show a diversity in developmental patterns that is so great that humans have great difficulty identifying a given species through its complete life cycle.
• Only a tiny fraction of all insect species has ever been reared through a single generation.

Importance of arthropods to man

1. Beneficial insects
2. Insect pollinators e.g. bees, butterflies.
3. Commercial products derived from insects e.g. honey, beeswax, silk and dyes.
4. Entomophagous insects used in biological control e.g. Braconidae, Ichneumonidae and Chalcidae,
5. Insects as scavengers remove unwanted materials, thus cleaning the environment. E.g. scarab beetles.
6. Insects also serve as destroyers of undesirable plants.
7. Insects serve as food for people and animals e.g. locusts and termites.
8. Insects used in surgery and medicine e.g. acanthadrin from blister beetles.
9. Insects are used in scientific research e.g. Drosophila and
10. Injurious insects
11. Insects attacking cultivated plants
    • Plant injury by feeding
    • Plant injury by oviposition
    • Insects that spread plant diseases.
12. Insects attacking stored products
    • Pests of wood
    • Pests of fabrics and clothing
    • Pests of stored food
13. Insects attacking humans and animals
    • Annoyance of insects
    • Venomous insects
    • Parasitic insects
    • Insect disease vectors.

REFERENCES

Invertebrate Zoology by E. E. Ruppert, R.S. Fox, and R.D. Barnes. 7^th ed. Thomson. R. S.K. Barnes, P. Calow, P.J.W. Olove, D.W. Golding, J.L. Spicer (Eds).

Invertebrate Zoology: A laboratory manual. 6^th Ed. Wallace, R.L. & W.K. Taylor. Prentice Hall, N.J.

Invertebrate Structure and Function. Barrington, E.J.W. Thomas Nelsonanmd Sons Ltd, London.

Living Invertebrates. Pearse and Buchsbaum.