Phylum Mollusca is a diverse group of invertebrates characterized by a soft body, often protected by a hard shell. Mollusks have a muscular foot for movement, a mantle that secretes the shell, and a radula for feeding (in most classes). They inhabit marine, freshwater, and terrestrial environments.
General Characteristics:
1) Habitat: Majority of the members are aquatic, mostly marine, few freshwater and some are terrestrial.
2) Organisation: These species exhibit organ system level of organisation.
3) Germ Layer: They are triploblastic animals.
4) Symmetry: They usually show bilateral symmetry. In some molluscs due to torsion, they become asymmetrical.
5) Coelom: They are coelomate animals.
6) Digestion: Digestive system is complete with mouth and anus. Digestion is extracellular.
7) Respiration: Respiration through molluscan gills or Ctenidia (in aquatic forms) and Pulmonary sac (in terrestrial form) helps in aerial respiration. Haemocyanin is the respiratory pigment.
8) Excretion: Excretory organs are one/two pairs of kidney. Gills are also excretory in function. Ammonia is the excretory matter.
9) Circulation: Circulatory system is open type by haemocoel.
10) Locomotion: A ventral muscular foot is present.
11) Reproduction: They are unisexual but few are hermaphrodite. Fertilization may be external or internal.
12) Development: Development is either direct or indirect. If indirect, then larva present- trocophore, glochidium.
13) Shell: Shell is secreted by mantle made of CaCO3. Shell may be external, internal (cuttle fish) or absent (octopus).
14 Mantle (Pallium): Mantle is thin, fleshy fold of dorsal -body covering the body wall. It enclose a space i.e. mantle cavity.
15) Nervous system: Nervous system consist of paired cerebral, buccal, pleural and pedal ganglia and visceral ganglia.
Example: Pila globusa (Apple Snail); Helix sp. ; Pinctada sp.; Octopus vulgaris (Octopus); Loligo sp.(Squid) etc.
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