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Phylum Mollusca

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Phylum Mollusca CLASS XI [BIOLOGY] PHYLUM MOLLUSCA General Characters 1. Habitat : They are mostly marine. Many, however, occur in fresh water and some even in damp soil. 2. Body form : The body of molluscs is unsegmented with a distinct head, muscular foot and visceral hump. 3. Symmetry : They usually show bilateral symmetry. In some molluscs like Pila, due to torsion (twisting) during growth, the adults become asymmetrical. 4. Shell : Shell is secreted by mantle. It is made up of calcium carbonate. Shell may be external (e.g. most of molluscs), internal (e.g., slug, cuttle fish, squid) or absent (e.g. Octopus) 5. Mantle (Pallium) : It is a thin, fleshy fold of dorsal body wall more or less covering the body. It encloses a space, which is called mantle cavity ( = pallial cavity) 6. Body wall : Single layered epidermis is usually ciliated. Muscles are unstriped and occur in bundles. 7. Body cavity : Coelom is greatly reduced. It is restricted to pericardial cavity (space around the heart), and to small spaces within kidneys and gonads (testes and ovaries). Spaces amongst the viscera (soft organs) contain blood and form haemocoel. 8. Digestive tract : It is complete. 9. Blood vascular system : It is open type. It includes dorsal heart, arteries that open into sinuses (spaces) and veins. Blood is usually blue due to the presence of a copper containing blue respiratory pigment called haemocyanin. Among the molluscs, cuttle fish are exceptional in having closed blood vascular system. 10. Respiratory organs : These are gills (ctenidia), mantle and pulmonary sac (in semi terrestrial form). 11. Excretory organs : Excretory organs are one or two pairs of sac- like kidneys, Gills are also excretory in function. Ammonia is chief excretory matter. 12. Nervous system : The nervous system comprises paired cerebral, pleural, pedal and visceral ganglia joined by the nerve connectives and commissures. Connectives connect dissimilar ganglia, however, commissures connect similar ganglia. 13. Sense organs : In many molluscs, eyes are present over stalks called ommatophores. Statocysts (balancing organs) may be present. Osphradium is present in some molluscs for testing chemical and physical nature of water. 14. Sexes : The sexes are generally separate but some are hermaphrodite. 15. Development : They are oviparous. The development is either direct or indirect (metamorphosis). When the development is indirect, it includes a characteristic larva, veliger, trochophore or glochidium. Asexual reproduction is absent. Unique Features : (i) Mantle covers the body (ii) In some forms, a lung is present for pulmonary respiration (iii) Better developed sense organs such as eyes, statocysts and osphradia. Pila (Apple snail) : It inhabits ponds, paddy fields, sometimes streams and rivers. It is chiefly herbivorous and feeds on aquatic plants like Pistia and Vallisneria. It leads an amphibious life respiring by means of gill in water and by a pulmonary sac an land. Thus it is adapted for both aquatic and terrestrial life. The mouth or aperture of the shell is closed by a flat and oval plate, the operculum. Pila has osphradium which is meant for testing chemical and physical nature of water. The buccal cavity of Pila contains a rasping organ, the radula, with transverse rows of teeth for cutting the grasses. Sepia — (Cuttle fish) : It is a marine and a good swimmer. In male the left arm is spoon shaped and is called hectocotylized which is used to transfer sperms into the female. The shell is internal. A pear-shaped ink-sac containing the ink-like fluid is present. When the animal is attacked ink- like fluid is ejected through the funnel to form a smoke cloud. Sepia is edible. Sepia ink obtained from this animal is used by artists. The shell of sepia is used as a source of calcium for pet birds. Loligo (Squid) : Like Sepia, it is also found in the warm seas. It also ejects a dark ink to form a smoke cloud to escape from the enemy. Its body resembles that of Sepia in form except that it is narrower than that of Sepia. It also has internal shell. Largest living invertebrate is giant squid (Architeuthis) upto 18 meters long, Loligo is edible. The giant squid has the largest eye in the animal kingdom. Octopus (Devil fish) : It is also marine. It kills its prey with poisonous saliva. One of the arms of male is spoon shaped and is called hectocotylized which is used to transfer sperms into the female. The shell is absent. Octopus ejects an inky fluid in water and forms a screen for defence from its enemies. Octopus can change its colour. * * * .
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