Phylum Mollusca (General characteristics and classification)

General features:

  • They are commonly called soft bodied animals.
  • They are mostly marine, few fresh water and also found in damp soil.
  • It is the second largest phylum in the animal kingdom.
  • Body is unsegmented (except Neoplina) and bilaterally symmetrical (except few twisting forms like Pila).
  • They have organ system grade of body organization.
  • Body is triploblastic and haemocoelomte.
  • Body is divided into three distinct parts; foot for locomotion, head bears sense organs and visceral mass (digestive and circulatory organs).
  • Body is covered by calcareous shell that may be external (e.g. Pila), internal (e.g. slug, sea hare, sepia) and absent (e.g. sea lemon, octopus).
  • Glandular fold of the body is called mantle or pallium.
  • Mantle cavity or pallial cavity is located between shell and visceral mass.
  1. Body wall:
  • It consists of single layered epidermis which is ciliated.
  • Muscles are unstriped and occur in a bundle.
  1. Digestive tract:
  • Alimentary canal is complete and the mouth is guarded by operculum attached to foot.
  • In some animals like Pila gut is provided with rasping organ called radula and odontophore with rows of 7 teeth.
  1. Respiratory organs:
  • Aquatic forms respire through comb like gills called ctenidia whereas terrestrial forms respire through pulmonary sac.
  1. Sense organs:
  • Oshphradium (olfactory organ) are present which that test the chemical nature of ingoing water current.
  • Eyes act as photoreceptors, located on the tip of the tentacles.
  • Statocysts are the balancing organs. Tentacles are tactile organs.
  1. Circulatory system:
  • It is generally of open type except cephalopods.
  • The heart is myogenic and dorsally
  • Blood is blue in color due to the presence of copper containing respiratory pigment called Hemocyanin.
  1. Excretory system:
  • They are ammonotelic.
  • One or two pairs of sac like kidneys or organs of Bojanus are present.
  • Gills are also excretory in function.
  1. Nervous system:
  • It consists of brain, paired cerebral, pleural, pedal, stellate and visceral ganglia that are joined by nerve connectives and commisures.
  1. Reproduction:
  • Sexes are generally separate, but some are hermaphrodite.
  • Reproduction takes place sexually and development may be direct or indirect.
  • Larva can be velliger, trochophore or


Phylum Mollusca has been divided into following six classes on the basis of foot, shell and mantle.

Class: Monoplacophora

  • They are marine.
  • Body is segmented and bilaterally symmetrical.
  • Foot is flat and ventral.
  • Shell consists of single piece or valve.
  • Head is without eyes and tentacles.
  • Mantle is present which encircles the body as a circular fold of the body wall.
  • e.g. Neoplina (connecting link between Annelida and Mollusca).

Class: Aamphineura

  • They are mostly marine.
  • Body is elliptical, convex dorsally and flattened ventrally.
  • Foot is flat and ventral.
  • Shell consists of 8 plates.
  • Head is distinct but without eyes and tentacles.
  • Development is indirect with trochophore larva.
  • Mantle is present.
  • e.g. Chiton

Class: Scaphopoda

  • They are marine.
  • Body is elongated and enclosed in a tusk like shell.
  • Foot is conical for digging.
  • Shell is cylindrical and the head bears a mouth.
  • Development is indirect with a trochophore
  • Mantle is single lobed and tubular.
  • e.g. Dentalium

Class: Gastropoda

  • They are either aquatic or terrestrial.
  • Body is unsegmented and asymmetrical.
  • Foot is sole like, broad, flat and muscular.
  • Shell is univalve, hence also called Univalvia.
  • Head is distinct with tentacles, eyes and mouth.
  • Development is indirect with dorsally located or velliger
  • Mantle helps in respiration.
  • e.g. Helix (Snail), Limax (Slug), Pila (Apple snail)

Class: Pelecypoda

  • Body is laterally compressed.
  • The foot is tongue
  • Shell is bi-valve and hence also called Bivalvia.
  • Head is not distinct.
  • Development is indirect with glochidium
  • Mantle is paired and consists of right and left lobes.
  • They are commonly called pearl forming groups that is secreted by the shell gland of mantle.
  • e.g. Unio, Oyster

Class: Cephalopoda

  • They are mostly marine.
  • Body is bilaterally symmetrical with head and trunk.
  • Foot is modified into arms and siphon.
  • Foot is situated on the head.
  • Shell is absent or internal.
  • Head bears large eyes (resembles with that of vertebrates) and mouth.
  • Circulatory system is closed type.
  • Mantle may be present or absent.
  • Development is direct.
  • e.g. Sepia, Octopus, Loligo

Phylum Mollusca (General characteristics and classification)