- BS Media
- Published: 05, Jun, 2017 | Updated: June 05, 2017
- They are exclusively marine.
- They are free living. Some are pelagic. Some are creeping on the sea bottom.
- These are all triploblastic and co-elomate animals.
- Adults show pentaradial symmetry and larval form show bilateral symmetry.
- The size varies from moderate to considerable size.
- Shape of the body is spherical or globular or elongated. It shows oral aboral and sides.
- Exo and endo skeletons will be present.
- Endo-skeletoh is mesodermal. It is made by hard polygonal plants.
- Body walls include only Epidermis, dermis and peritonium.
- Body cavity or coelome is large. It is an entero-coelome. It is lined by ciliated peritonium.
- Respiration is carried on by
a) Papullae -Star fish
b) Peristomial gills - Sea urchin
c) Genital bursae - Brittle star
d) Respiratory trees - Holothurians
- Excretory organs are absent.
- Nervous system is primitive. Brain is absent.
- Special sense organs are poorly developed. Eye spots and statocysts are present.
- Reproduction is sexual. Sexes are separate. Sexual dimorphism is absent. Gonads are iarge.
- Gametes are iiberated into the water. Fertilisation is external.
- Eggs are homolethial. Cleavage is radial and indeterminate type.
- Life history includes larval stage.
- They can show autotomy and regeneration.
The classification followed here is based on Hyman's Classification.