Phylum Mollusca

Phylum Mollusca (Latin mollis = soft) includes animals such as Pila, bivalves, Octopus (devil fish), Sepia (cuttlefish), Chaetopleura (chiton), Pinctada (pearl oyster), Loligo (squid), Aplysia (sea hare) and Dentalium (tusk shell). It is the second-largest phylum in the animal kingdom.

Molluscs may be free-living or sedentary, and are mostly aquatic or found in marshy habitats, with a few terrestrial forms. They are soft-bodied and follow the tube within tube body plan. Most are bilaterally symmetrical, though a few appear asymmetrical due to torsion (a twisting of the body during development).

The body is divided into a head, a muscular foot (on the ventral side, used for locomotion) and a visceral mass, which is enclosed within a thick, muscular fold of the body wall called the mantle. The mantle secretes a hard, calcareous shell, which may be external, internal, or absent altogether depending on the group.

The digestive system is well developed and complete, with the mouth at the anterior end and anus at the posterior. Inside the buccal cavity is a unique rasping feeding organ, the radula, bearing transverse rows of fine teeth used to scrape food. Aquatic molluscs typically have numerous feather-like gills, called ctenidia, located within the mantle cavity (the space between the visceral mass and the mantle) and used for respiration; terrestrial forms may instead have lungs. …