(i) Body bilateral symmetrical with a dome shaped mantle. (ii) Flattened limpet-shaped shell with spirally coiled protoconch.
(iii) Foot broad and flat with 8 pairs of pedal retractor muscles. These animals possess some annelidan characters: as (i) 5 or 6 pairs of giIIs and nephridia show segmental arrangement. Presumably these molluscs are a connecting link, between annelids and molluscs. Example: Neoplina.
2. Class 2: Amphineura (Gr., amphi, both + neuron, nerve) (i) Elongated body with reduced head. (ii) Radula present. (iii) Shell as 8 dorsal plates or as spicules. (iv) Foot ventral large flat and muscular. (v) Non-ganglionated nerve ring around mouth, 2 pairs of interconnected nerve cord.
(vi) Fertilization external, larva trochophore. Example: Neomenia, Chiton, Chaetopleura etc. 3. Class 3: Scaphopoda (Gr., Scapha, boat + podos, foot) (i) Tusk shells. (ii) Body within a tubular shell open at both ends.
(iii) No head, mouth with tentacles, no eyes. (iv) Foot conical, radula present, no gills. (v) Kidneys paired, gonad single. (vi) Dioecious, larva trochophore. Example: Dentalium, Cadulus, Pulsellum etc. 4.
Class 4: Gastropoda: (Gr., gaster, belly + podos, foot) (i) They are mainly snails and slugs. (ii) Torsion of body occurs sometimes. (iii) Well developed head with eyes and tentacles. Radula is present. (iv) Foot large and flat. (v) Shell present or absent, univalve and usually coiled. (vi) Mostly marine, some freshwater or terrestrial.
Example: Patella, Pila, Doris etc. 5. Class 5: Pelecypoda: (Gr., Pelepus, hatchet + podos, foot) (i) No Head, tentacles, eyes, jaws and radula. (ii) Body enclosed in a bivalve shell and laterally compressed. (iii) Foot often hatchet-shaped and valve extending between mantle lobes. (iv) Mostly filter feeding.
(v) Usually dioecious, veliger or glochidium larva. (vi) (vi) Mostly marine, a few freshwater, Examples: Mytilus, Unio, Teredo, Lamellidens etc. 6. Class 6: Cephalopoda: (Gr. Kephale, head + podos, foot) (i) Body dorosoventrally elongated.
(ii) Shell external, internal or absent. (iii) Head distinct and large with well developed eyes. Foot as tentacles and siphon, radula present. (iv) Dioecious, development direct. (v) Marine and free swimming.
Example: Nautilus, Octopus, Sepia, Loligo etc.