Updated by: Arabinda Naik
The phylum Mollusca (Molluscs — soft bodied) includes the soft, unsegmented, eucoelomate animals. These are called molluscs and shelled animals. Mollusca is the second largest animal phylum and includes about 85.000 species.
Habit and Habitat Molluscs are mostly marine forms. Sepia, Octopus, Chiton. some are freshwater (e.g. Unio and Pila) and Some are also terrestrial forms (e.g., Land snails). Few molluscs arc parasites also, e.g. Glochidium larva, etc.
Symmetry These are generally bilaterally symmetrical, and some are asymmetrical due to torsion or twisting during growth.
Germ Layers and Organisation They are triploblastic an possess organ system level of organisation.
Body Form They have unsegmented, soft body covered by a calcareous shell, which is differentiated into head. muscular foot and visceral hump. Mantle is a soft. spongy thick fold of skin Over the visceral hump that secretes calcareous shell. The space between the hump and the mantle is called the mantle cavity in which feather like gills arc present.
Skeleton Molluscs generally have shell as exoskeleton. In Octopus, shell is absent.
Body Cavity The coelom is greatly reduced although they are eucoelomate. Locomotion The locomotory organ is muscular foot.
Digestion The digestive tract is complete. The mouth contains a rasping organ called radula with file like transverse row of chitinous teeth. Anus opens into the mantle cavity.
Respiration In terrestrial forms respiration through lungs. In aquatic forms, respiration occurs through feather-like gills or ctenidia.
Excretion: A pair of metanephridia (kidneys) or organs of Boianus or Keber’s organs are present.
Circulation Open type of circulation is present. The coelom is called haemocoel.
Sense Organs These have eyes. statocyst. tentacles for equilibrium and receptors for touch. smell and taste.
Nervous System Few Pairs Of ganglia like cerebral. visceral and pedal with nerves is present.
Reproduction: Molluscs reproduce sexually. Sexes are separate and mostly oviparous. Fertilisation is external or internal. Development is either direct or indirect with larval Stages like trochophore, glochidium and veliger. e.g.. Chaetopleura (chiton), Dentalium (elephant tusk shell),