1. Molluscs are essentially aquatic mostly marine, few freshwater and some terrestrial forms.
2. The body is soft, unsegmented, bilaterally symmetrical and consists of head, foot, mantle and visceral mass.
3. The body is clothed with one layered often ciliated epidermis.
4. Body is commonly protected by an exoskeletal calcareous shell of one or more pieces, secreted by the mantle.
5. Head is distinct, bearing the mouth and provided with eyes, tentacles and other sense organs except in Pelecypoda and Scaphopoda.
6. Ventral body wall is modified into a muscular flat or plough-like surface, the foot which is variously modified for creeping, burrowing and swimming.
7. Mantie or pallium is a fold of body wall that leaves between itself and the main body mass, the mantle cavity.
8. Visceral mass contains the vital organs of the body in a compact form taking the form of a dorsal hump or dome.
9. Body cavity is haemocoel. The true coelom is generally limited to the pericardial cavity and the lumen of the gonads and nephridia.
10. Digestive tract is simple with an anterior mouth and posterior anus but in gastropods,scaphopods and cephalopods the intestine becomes U-shaped bringing the anus to an anterior position.
11. Pharynx contains a rasping organ the radula except in Pelecypoda.
12. Circulatory system is open except in cephalopods which shows some tendency towards aclosed system.
13. Respiratory organs consist of numerous gill or etenidia usually provided with osphradium at the base. Lung is developed in terrestrial forms. Respiratory pigmentis usually haemocyanin.
14. Excretory system consists of a pair of metanephridia which are true coelomoducts and communicate from pericardial cavity to the exterior by nephridiopore.
15. Nervous system consists of paired cerebral, pleural, pedal and visceral ganglia joined by longitudinal and transverse connectives and nerves.
16. Sexes usually separate (dioecious) but some are hermaphroditic.
17. Fertilization is external or internal.
18. Development is either direct or with metamorphosis through the trochophore stage called veliger larva.