MOLLUSCA - GENERAL CHARACTERS AND CLASSIFICATION:Term "mollusca" was given by Johnston.Phylum mollusca includes soft bodied invertebrate animals such as snails, slugs, mussels, clams, oysters, tusk shells, squids, octopods and nautili etc.Mollusca is the second largest phylum after arthropoda.Study of molluscs is called malacology.Study of shells of molluscs is called conchology.General characters:-They are mostly marine, however, some fresh water and terrestrial forms are also available.The body of molluscs is unsegmented (except monoplacophora) with a distinct head, muscular foot and visceral hump. Foot is useful in locomotion.Dorsally visceral mass is covered by a thick sheet of skin called the mantle or pallium. It encloses a space called mantle or pallial cavity between itself and the body.Body is bilaterally symmetrical. In some molluscs such as pila the adults become asymmetrical due to torsion during growth.The body is covered by a shell made up of calcium carbonate and an organic substance called conchin. It is secreted by the shell glands of the mantle.The body cavity is haemocoel. True coelom is confined to the kidneys, gonads and pericardial space.Buccal cavity contains a "rasping structure" called radula [Radula is absent in bivalves.Respiratory organs are ctenidia (gills). Pulmonary sac and secondary branchiae occur in some species.Circulatory system is open type [closed type in cephalopods]. The blood flows through large "sinuses". Heart has generally two auricles and one ventricle. Respiratory pigment is haemocyanin [which contains copper].Excretion takes place by tubular metanephridia commonly called as kidneys. Glands of the pericardium secrete wastes into the coelom and nephridia passes them out.Each nephridium is connected with pericardial coelom through nephrostome. The other end opens into mantle cavity through nephridiopore.Nervous system is well developed with ganglia, commissures and connective.Special sense organs like osphradium, which tests the quality of water and the amount of sediment in it. These occur in the bivalves and gastropods. Other sense organs like tentacles and eyes are present. Statocystes occur in a majority of molluscs.They are mostly unisexual. Few are hermaphrodite.Cleavage is holoblastic and spiral.Development is direct or indirect. Modified trochophore larva is called veliger larva is present in the life history of many molluscs.Torsion is fundamental feature of gastropods. Torsion promotes stability in the adult.Classification of phylum mollusca :Phylum mollusca is divided into seven classes:1. Aplacophora, 2.Polyplacophora 3.Monoplacophora 4.Gastropod 5.Scaphopoda 6.Pelecypoda. and 7.Cephalopoda.

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