General characteristics of phylum Mollusca:

(a) They have a soft, bilaterally symmetrical body with little segmentation and without appendages.

(b) Body is divisible into an anterior head, a ventral foot and a dorsal visceral mass. The entire body is covered by a fold of thin skin called mantle, which secretes a hard calcareous shell of one or more pieces.

(c) Respiration occurs via gills, mantle or a "lung" of the mantle.

(d) Sexes are usually separate.

(e) They have an open circulatory system (except in cephalopods). The body cavity is haemocoel and the true coelom is reduced and restricted to the pericardial cavity and the lumen of the gonads and nephridia.

(f) Some species have sensory organs of touch, smell, taste and vision.

(g) They excrete via metanephridia or kidneys.

(h) The molluscs are aquatic, mostly marine with a few freshwater and terrestrial forms.

Examples:

(i) Class Gastropoda: Pila (apple snail)

(ii) Class Pelecypoda: Teredo (ship worm)

(iii) Class Cephalopoda: Octopus