Phylum Mollusca

Phylum Mollusca

- Basically bilateral and protostomial eucoelomate eumetazoans whose soft body (L., mollis or molluscum = soft) is unsegmented and enclosed within a skin–fold (mantle) which usually secretes a calcareous shell.

- About 80,000 existing and 35,000 extinct species known. Aquatic or terrestrial.

- Snails and mussels found along banks of freshwater bodies and the oysters, shanks (shoes conch–shell is commonly blown in religious ceremonies) and cowries of the sea are common molluscs.

Malacology or Conchology is the branch of study of molluscs and their shells.

Molluscs are of some economic importance; man uses some as food; shells of some are used for making buttons, beads, etc.

Brief History

Aristotle described a number of molluscs. Johnston (1650) proposed the name of the phylum.

Salient Features

(1) Molluscs are multicellular organisms

(2) They have a bilateral symmetry, but snails are asymmetrical

(3) They are triploblastic animals.

(4) They are coelomate animals. True coelom is reduced the haemocoel is well developed in them.

(5) They have organ system grade of organization.

(6) The body is soft and unsegmented.

(7) The soft body is covered by a fleshy fold of the body wall. It is called mantle.

(8) The molluscs are provided with one or two calcareous shells. The shells may be external or internal.

(9) Respiration is carried out by the gills or pulmonary chambers.

(10) The digestive system is well developed. It contains a radula and a hepatopancreas.

(11) The circulatory system is of an open type.

(12) The excretory organ is the kidney.

(13) The nervous system is well developed.

(14) The sensory organs are eyes, statocysts and osphradia.

(15) Sexes are separate in them, or they are hermaphrodites.

(16) The development in their case in either direct or indirect

Classification of Mollusca

Class 1. Aplacophora or Solenogasters

(1) The body is worm–like, bilaterally symmetrical and cylindrical.

(2) The head, mantle, foot, shell and nephridia are absent.

(3) The body is covered with spicule–bearing cuticle.

(4) The digestive tract is straight with radula.

(5) A mid dorsal longitudinal keel or crest is often present .

(6) Example : Neomenia, Chaetoderma, etc.,

Class 2. Monoplacophora

(1) The body is bilaterally symmetrical and segmented.



(2) The shell is formed of a single valve.

(3) The head is without eyes and tentacles.

(4) The gills are external and serially arranged.

(5) The nephridia are five pairs.

(6) Example : Neopilina galatheae






Neopilina is a living fossil and connecting link between Annelida and Mollusca.


Dentalium is commonly called tusk shells.


Pearl oyster belongs to the class pelecypoda.


Echinoderms also known as Floating stone.






Class 3. Polyplacophora

(1) These molluscs are bilaterally symmetrical, and dorsoventrally flattened.

(2) The shell is composed of a longitudinal series of 8 plates.

(3) The foot is flat and ventral.

(4) The radula is well developed.

(5) Example : Chiton, Cryptochiton, etc.

Class 4. Gastropoda

(1) It seems that these animals are moving on their stomach. Hence the name gastropoda.

(2) Gastropods are marine, fresh water or terrestrial animals. A few are parasitic.

(3) The body is unsegmented and asymmetrical.

(4) The shell is univalve and spirally coiled.

(5) The head is distinct. It bears tentacles, eyes and a mouth.

(6) The foot is ventral and muscular.

(7) The buccal cavity is provided with a radula.

(8) The circulatory system is open.

(9) The sexes are mostly separate, while some forms are hermaphrodite.

(10) The development includes veliger and trochophore larvae.

(11) Examples : Haliotis, Cypraea (Cowrie) Pila (apple snail), Murex (rock shell) Aplysea (sea hare etc), Doris (Sea Lemon)

Class 5. Scaphopoda

(1) The foot is boat–shaped.

(2) The eyes, the tentacles and ctenidia are absent.

(3) Marine, bilaterally symmetrical molluscs.

(4) Examples : Dentalium, Siphonodentalium and Pulsellum

Class 6. Pelecypoda

(1) Pelecypoda are aquatic in habit.

(2) The body is bilaterally symmetrical and laterally compressed.

(3) The shell is formed of two distinctive shell plates.

(4) The head is not distinct.

(5) The alimentary canal shows a crystalline style.

(6) The gills, excretory organs and the other structures are paired.

(7) The sexes are separate or united.

(8) The development is indirect having a glochidium larva.

(9) Example : Mytilus, Unio, Tredo.

Class 7. Cephalopoda

(1) The body is bilaterally symmetrical.

(2) The foot is modified into arms and funnel.

(3) The shell may be either absent or rudimentary; it may be internal or external.

(4) The odonotophore with a radula is present.

(5) The ink–gland is present.

(6) The sexes are separate.

(7) The development is direct hence no metamorphosis and larval stage.

(8) Example : Nautilus, Loligo Sepia, Octopus.