Phylum : Echinodermata

The name of the phylum is derived from the presence of an endoskeleton of calcareous ossicles (Greek: echinos– hedgehog, derma– skin).

Habitat: All echinoderms are exclusively marine.

Body plan: The members have organ-system level of organisation.

Examples:


Asterias (Star fish) Echinus (Sea urchin) Antedon (Sea lily)

Cucumaria (Sea cucumber) Ophiura (Brittle star)

Symmetry: The adult echinoderms show penta-radial symmetry but larvae are bilaterally symmetrical.

Germ layer: They are triploblastic and coelomate animals. Body wall has three layers – an outer epidermis, a middle dermis and an inner peritoneum.

Body characteristics:

  • They do not have a distinct cephalization.
  • The most distinctive feature of echinoderms is the presence of water vascular system which helps in locomotion, capture and transport of food and respiration. It is developed from a part of the embryonic coelom.
  • Respiratory structures vary in different members.
  • Papillae in Starfish.
  • Peristomial gills in sea urchin.
  • Genital bursae in brittle star.
  • Cloacal respiratory tree in Holothurians.

Physiology:

  • Digestive system is complete with mouth on the lower (ventral) side and anus on the upper (dorsal) side.
  • An excretory system is absent.
  • Nervous system and sensory organs are poorly developed.

Reproduction: Sexes are separate. Reproduction is sexual.

Embryo Development: Fertilization is usually external. Development is indirect with free-swimming larva.

 

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