Quiz 16: Molluscs
The impact of the phylum as Mollusca on humans can be beneficial in the following ways: • Many kinds of mollusks are used as food • Pearl buttons are obtained from shells of bivalves • both natural and cultured pearls are produced in the shells of clams and oysters The harmful impact of the phylum Mollusca on humans are: • they are considered pests because of the damage they cause • Burrowing shipworms, which are bivalves of several species, do great damage to wooden ships and wharves. • Snails and slugs frequently damage gardens and other green areas • Snails also serve as intermediate hosts for serious parasites of humans and domestic animals. • Snails also destroy oysters.
A molluscan coelom develops by spiral mosaic cleavage and makes a coelom by the process of Schizocoely. Evolutionary development of coelom was important because it enabled better organization of visceral organs in the body cavity and allowed the alimentary canal to become more specialized and more diversified without the interference of other organs.
The distinguishing characteristics of phylum Mollusca are: • They have a dorsal body wall which forms a pair of folds called mantle, which encloses the mantle cavity, is modified into gills or lungs, and secrete shell • The ventral body wall is specialized as a muscular foot, mainly used for locomotion; • They live in marine, freshwater, and terrestrial habitats. • It is Free-living; occasionally parasitic. • It is bilaterally symmetrical • It has a triploblastic body. • Surface covered with cilia and has mucous glands and sensory nerve endings • It has a complex digestive system with the rasping organ (radula) usually present; anus usually empties into mantle cavity • The body wall contains circular, diagonal and longitudinal muscles • Their Nervous system has paired cerebral, pleural, pedal, and visceral ganglia, with nerve cords and subepidermal plexus; the ganglia are centralized in the nerve ring in gastropods and cephalopods. • Sensory organs of touch, smell, taste and equilibrium present • No asexual reproduction is seen. • Both monoecious and dioecious forms are present • spiral cleavage is seen • Ancestral larva is a trochophore, many with a Veliger larva, some with direct development. • One or two kidneys (metanephridia) opening into the pericardial cavity and usually emptying into the mantle cavity. • Gaseous exchange by gills, lungs, mantle, or body surface. • Open circulatory system of heart (usually three-chambered), blood vessels, and sinuses; respiratory pigments in blood.