These are bilaterally symmetrical animals with a reduced coelomic cavity. They have very little segmentation and organ system level of organisation. They have three layers of cells; they are known as triploblastic animals.
Other characteristics:
  • The body of these animals is divided into head, a visceral mass and muscular foot.
  • Few molluscs have a hard external shell similar to the snails.
  • They have an open circulatory system.
  • They have a kidney-like organ for excretion.
Chiton, Octopus, Pila, Unio
Phyllum mollusca
The term Echinodermata is derived from the Greek word echios which refers to hedgedog (a spiny mammal) and derma that refers to skin. These are spiny skinned free-living marine organisms.
Body characteristics:
  • They are triploblastic coelomic animals.
  • They are bilaterally symmetrical in their larval stage and radially symmetrical in the adult stage.
  • Their body lacks segmentation and definite organs.
Phyllum echinodermata