MOLLUSCA: CHARACTERS AND CLASSIFICATION

The animals of this phylum are soft-bodied, non-metameric, triploblastic coelomates and fundamentally bilaterally symmetrical invertebrates with a thin fleshy envelope the mantle, around the visceral (internal organs of the body) which may secrete calcareous shell. They are sluggish animals. General characters Widely distributed over the world. Mostly aquatic (marine and few are fresh-water) and some are terrestrial living in damp soil. Body is soft and unsegmented (except Neopilina). Bilaterally symmetrical except gastropodas (asymmetrical) because the shell is coiled. Schizocoel or Haemocoel cavity found as coelom. Triploblastic Organ system level of body organization. Body is divisible into head (absence in pelecypoda and scaphopoda), dorsal visceral mass, ventral muscular foot and mantle. Mantle- is a dorsal glandular fold of the body wall. It is thick and muscular and encloses a mantle cavity. Head- anterior part of the body, which contains mouth, eyes and tentacles and other sense organs. Foot-is ventral in position usually thick and muscular and generally forms the main locomotory organs. Visceral mass- on dorsal side contains digestive, circulatory and reproductive organ of the body and it forms a hump or dome. Body is covered by a hard calcareous structure, made up of calcium carbonate, called shell. In some molluscus, it is internal or reduced or even absent (oplacophora). Locomotion takes place by ventral muscular foot. Digestive system is well developed with a hard chitinous structure, called radula. Respiration takes place through one or more gills or ctenidia, lungs (pulmonary sac) or general body surface in the terrestrial forms. Circulatory system is closed or open type. Head consists tentacles and compound eyes. Presence of one pair tentacles except octopus where tentacles are modified into arm. Excretion takes place by paired metanephridia (kidney). Nervous system consists of many paired ganglia, connectives and nerves. Sense organs are eyes, tentacles, osphradium and statocyst. Sexes are usually separate but some are monoecious. Pila- sexual dimorphism. Limax- hermaphrodite Helix- hermaphrodite Fertilization is external (in sessile group such as in oyster) or internal. Development may be direct or indirect. Larva is trochopore or veliger or parasitic stage called glochidium larva.