Give the general characters of phylum mollusca and list the distinguishing features of the classes under it.

Explanation

The general characters of phylum mollusca are as follows:
(i) The body is usually unsegmented except Neopilina.
(ii) The symmetry is bilateral in phylum mollusca
(iii) The mollusca are triploblastic animals and have organ-system level of organisation.
(iv) The body is divisible into three regions : anterior head, dorsal visceral mass and ventral foot.
(v) In this phylum, a thin fold of dorsal body wall covers the body. This fold is called the mantle.
(vi) The coelom of mollusca is greatly reduced. It is schizocoel.
(vii) The digestive tract is complete. A rasping organ, the radula, is present in the buccal cavity. viilIn mollusca, the respiration occurs through gills, called ctenidia, found in the mantle cavity.
(ix) The circulatory system is open in mollusca. Blood contains a copper-containing, blue respiratory pigment, called haemocyanin / plastocyanin.
(x) Excretion occurs with the help of one or two pairs of sac like kidneys.
(xi) The nervous system contains three pairs of ganglia, cerebral, pedal and visceral. These are inter-connected by commissures and connectives.
(xii) In phylum mollusca, sexes are generally separate. Gonads have ducts. Fertilization can be external or internal. Asexual reproduction is absent.