(i) This is the second largest animal phylum.
(ii) Molluscs are terrestrial or aquatic (marine or fresh water)
(iii) Organization : organ system level.
(iv) They are bilaterally symmetrical (except univalves), triploblastic and coelomate animals.
(v) Body is covered by a calcareous shell Unsegmented with head, muscular foot and a visceral hump or visceral mass.
(vi) Mantle cavity - The space between the visceral mass and mantle (pallium) is called the mantle cavity.
(vii) Feather like gills (ctenidia) are present, which are respiratory in function.
(viii) The digestive system : complete and mouth contain radula with transverse rows of chitinous teeth for feeding (radula is absent in bivalves).
(ix) Sense organs : tentacles, eyes and ospharidium (test the purity of water)
(x) Excretory organs are nephridia.
(xi) Circulatory system : open type except squids, cuttle fishes and octopuses. Blood contains haemocyanin, a copper containing respiratory pigment.
(xii) Development is indirect with a veliger larva. eg. Pila (Apple snail).
