Classification of Phylum Mollusca according to Ruppert & Barnes (1994)

Phylum :- Mollusca

Characters :-

(i) Bilaterally symmetrical, body soft, unsegmented, without a body cavity.
                 
(ii) Presence of a protective external dorsal shell. In some cases, it may be internal & covered by tissue or secondary reduced or lost.
                 
(iii)A visceral mass remains enclosed by a thick muscular fold of the body wall called mantle.
                 
(iv)Body monomeric & highly variable, essentially with an anterior head bearing eyes & sensory tentacles, a large ventral foot & posterior mantle cavity.

Class :- Aplacophora

Characters :-

(i) Worm like mollusca, body covered by cuticle, absence of shell.
                 
(ii) Foot absent or reduced to a ventral ridge.
                 
(iii) Head poorly marked & without eyes or sensory tentacles.
                 
(iv) There are no excretory organs.



Examples:- Chaetoderma; Neomenia; Proneomenia;

Fig :- Chaetoderma

Fig :- Neomenia

Class :- Polyplacophora

Characters :-

(i) Dorsal surface of body covered by eight shell plates.
                 
(ii) A broad muscular creeping foot is present on the ventral surface.
                 
(iii) Head is poorly developed. Eyes & tentacles is absent.
                 
(iv) Radula is large & bears many teeth.



Examples:- Chiton; Chaetopleura; Loricata;

Fig :- Chiton

Fig :- Loricata

Class :- Monoplacophora

Characters :-

(i) Bilaterally symmetrical body & covered by a shield-like shell.
                 
(ii) On the ventral side is a weakly muscular, circular foot surrounded laterally & posteriorly by an extensive mantle cavity.
                 
(iii) Head distinct but poorly developed & without sensory tentacles.



 Examples:- Neopilina; Monoplacophorus;

Fig :- neopilina

Fig :- Monoplacophorus

Class :- Gastropoda

Characters :-

(i) Visceral mass is twisted at 180 degree in an anticlockwise direction(torsion), relative to the head & foot.
                 
(ii) Shell is one piece & asymmetrically spiraled.
                 
(iii) A muscular foot is present below the digestive system & visceral mass.
                 
(iv) Head distinct with one or two pairs of tentacles & eyes. In the digestive system, buccal contain a scrapping radula.



Examples:- Pila; Patella; Aplysia; Achatina; Helix; Lymnaea; Limax; Doris; Murex;

Fig :- Aplysia

Fig :- Pila

Class :- Bivalvia

Characters :-

(i) Laterally compressed body enclosed within a pair of shell valves.
                 
(ii) Head indistinct & a radula is absent.
                 
(iii) Mouth provided with two pairs of labial palps.
                 
(iv) Relatively sedentary or even sessile.



Examples:- Mytilus; Pinctada; Lamellidens;

Fig :- Mytilus

Fig :- Picntada maxila

Class :- Scaphopoda

Characters :-

(i) Presence of tubular like snell,open at both ends.
                 
(ii) The elongated body completely enclosed by the mantle.
                 
(iii) The small proboscis like head lacks & sensory tentacles.



Example:- Dentalium;

Fig :- Dentalium

Class :- Cephalopoda

Characters :-

(i) Foot modified into a series of prehensile arms or tentacles & siphon.
                 
(ii) Shell absent or reduced & covered by the mantle in most species.
                 
(iii) This group has a well formed head bearing large eyes.
                 
(iv) Mouth possesses radula & jaws.



Examples:- Sepia; Loligo; Octopus;

Fig :- Octopus

Fig :- Loligo