Phylum: Mollusca

This phylum forms the second largest animal group in kingdom Animalia.

Habitat: They can be terrestrial or aquatic.


Pila (Apple snail) Pinctada (Pearl oyster) Sepia (Cuttlefish)

Loligo (Squid) Octopus (Devil fish) Aplysia (Seahare)


Dentalium (Tusk shell) Chaetopleura (Chiton)

Body plan: They have organ-system level of organization.

Symmetry: Molluscans are initially bilaterally symmetrical, but later due to torsion; it is lost in most of the members.

Germ layer: They are triploblastic and coelomate animals.

Body characteristics:

  • Body is divided into distinct head, muscular foot and visceral hump and a mantle.
  • The mantle is composed of a soft and spongy layer of skin over the visceral hump. It secretes a shell which later hardens by the deposition of calcium.
  • The space between the hump and the mantle is called the mantle cavity. Gills are present in this cavity.
  • The anterior head region has sensory tentacles.
  • The mouth contains a file-like rasping organ for feeding, called radula.


Physiology: Gills have respiratory and excretory functions. Annelids have an open circulatory system. The body fluids contain phycocyanin (a copper- containing pigment).

Reproduction: Sexes are separate (dioecious). Fertilization is oviparous.

Embryo Development: Development is indirect involving 3 larvae- Trochophore larva, Veliger larva and Glochidium larva.


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