Molluscs

Molluscs

Molluscs are soft-bodied invertebrates that are partially or wholly enclosed in a shell composed of calcium carbonate which is secreted by a mantle covering the body. The soft body consists of the head and a tail. They can survive in any habitat except in air. Their geographical distribution is affected by the close association with food. They are the most popular in the kingdom Animalia. Molluscs are in the phylum Mollusca and has many species. Besides, Mollusks are important members of the ecosystem and also in the food chain. They have medical and commercial importance to human beings. Mollusks nervous system helps mostly in sensation and varies from one group to another. There are many Mollusk groups such as Gastropods, Bivalves, and Cephalopods (Sigwart, & Sumner-Rooney, 2015). This paper will explore the differences and similarities of the nervous systems between two groups of Mollusks.

1. Comparison and contrast between the Gastropods and Bivalves nervous system

Contrast

The nervous system of the Mollusks is comprised of the nerve cords, a pair of ganglia, and eyes. However, there are many differences and similarities from one group to another. First, the nervous system of Gastropods is made of a series of ganglia which is in pair and is connected by nerve cords. Gastropods have six sets of ganglia which makes their nervous system complex. Their tentacles are located on the head and contain sensory organs. Gastropods have a brain which is located next to the esophagus and consisting of three pairs of ganglia. In other species, the ganglia are moderately discrete and in some, they are closely bound to form separate lobes of a single structure. In contrast, Bivalvia for example oysters, mussels, and clams have a simple nervous system with three sets of ganglia which are united by nerve fibres. The sedentary lifestyle makes bivalves’ nervous system less complex. Bivalves have no brain and the nervous system comprises of a series of paired ganglia and nerve network. Most species have two cerebropleural ganglia which are on the esophagus side. Bivalves have no brain because they lack a head region hence their eyes and tentacles are located in the mantle edge (Pavlicek, Schwaha, & Wanninger, 2018).

Furthermore, Bivalvia have a decentralized nervous system which is bilaterally symmetrical consisting of cerebral ganglia, visceral ganglia, and pedal ganglia. Adult bivalves’ nervous system is tetraneural comprising of two conspicuous pairs of longitudinal neurite bundles which are ganglionated differently (Wanninger, 2015). Gastropods have both central and peripheral nervous system with the central nervous system comprising of ganglia connected by nerve cell. The ganglia include paired pedal ganglia, cerebral ganglia, parietal ganglia, osphradrial ganglia, visceral ganglia and pleural ganglia. The neurons in mature gastropods especially freshwater snails help in food hunting. One neuron helps in detective presence of food while the other neuron senses whether the snail is hungry for feeding (Gillette, & Brown, 2015).

Also, the third pair of gastropods ganglia inside the brain lies somewhat behind and beneath the cerebral ganglia. The third pair is pleural ganglia which supplies mantle cavity nerves. Bundles of nerves hook up the pedal, cerebral, and pleural ganglia collectively and running below and above esophagus to unite left and right pedal and cerebral ganglia together (Chen et al., 2015)r. Whereas, the pleural ganglia of the Bivalves supply the mantle cavity nervous while cerebral ganglia have control on the sensory organs. Pedal ganglia are at the base and controls the feet. The visceral ganglia are beneath posterior abductor muscle. Nerve fibres connect ganglia to the cerebropleural ganglia (Wanninger, 2015).

Additionally, gastropods have single siphon while some bivalves have siphons which are paired and may contain siphonal ganglia. The siphons are located in along the mantle to allow water flow thus aiding the species in feeding, respiration and locomotion. The gastropods with siphons include sea snails while bivalves with siphons include mussels. The siphon in gastropods has chemoreceptors for smelling and tasting to aid in hunting food (Chen et al., 2015). Finally, some Bivalvia such as scallops and Clams have chemosensory organs and simple eyes situated along the periphery of the mantle. The organs initiate decision making through information integration, therefore, helping them escape in case of danger. Gastropods have specialized and diverse sensory organs than Bivalvia. They are active and motile hence can detect danger easily (Gillette, & Brown, 2015).

Similarities

Both bivalves and gastropods have the nervous system which performs functions which are associated with sense organs. In both, the central nervous system consists of cerebral, pleural, and pedal ganglia which innervate different organs of the body. Besides, some of the bivalves have eyes such as scallops have complex eyes with a two-layered retina, lens, and concave mirror just like in most gastropods where most o them have simple visual organs ranging from simple oceilli to more complex pit eyes with lens. The gastropods species with complex eyes include freshwater snails. Finally, both classes have sensory organs which are mechanoreceptors and chemoreceptor for sensation, taste, and touch which enables them to survive in their habitat (Pavlicek, Schwaha, & Wanninger, 2018).

1. Why similarities and differences make evolutionary sense

Mollusks exhibit a remarkable morphological difference such as in bivalves and gastropods. The phenotypic variation renders Mollusks best for studies into animal evolution. There are modifications which have taken place over the years that make different classes possess differences and similarities in the nervous system. The modifications include attainment of deeper mode of nervous coordination, concentration of the whole system, complexities due to torsion and detorsion like in gastropods. For example, in gastropods, the nervous system is detorted with supraintestinal ganglion moving to fuse with the right pleural ganglion (Hochner, & Glanzman, 2016).

In conclusion, Mollusks are soft-bodied invertebrates that are partially or wholly enclosed in a shell. The Bivalvia and Gastropoda classes have many differences starting with the presence of the brain, central nervous system, eyes, and some pairs of ganglia. They also share some characteristics in the nervous system in function like sensation. The evolutionary sense is made from the continuous change in the complexity of the animal classes concerning their lifestyle.