Write the general characters of the phylum mollusca.

Explanation

(i) This is the second largest animal phylum.
(ii) Molluscs are terrestrial or aquatic (marine or fresh water) and exhibit organ system level of body organisation.
(iii) They are bilaterally symmetrical (except univalves), triploblastic and coelomate animals.
(iv) Body is covered by a calcareous shell and is unsegmented with a distinct head, muscular foot and a visceral hump or visceral mass.
(v) A soft layer of skin forms a mantle over the visceral hump. The space between the visceral mass and mantle (pallium) is called the mantle cavity.
(vi) A number of feather like gills (ctenidia) are present, which are respiratory in function.
(vii) The digestive system is complete and mouth contains a rasping organ called radula withrows of chitinous teeth for feeding (radula is absent in bivalves.
(viii) The sense organs are tentacles, eyes and ospharidium (to test the purity of water and present in bivalves and gastropods).
(ix) Excretory organs are nephridia.
(x) Open type of circulatory system is seen except for cephalopods such as squids, cuttle fishes and octopuses. Blood contains haemocyanin, a copper containing respiratory pigment.
(xi) Development is indirect with a veliger larva.
(xii) eg. Pila (Apple snail).