Phylum EchinodermataGENERAL CHARACTERS
HABIT AND HABITAT
The Echinodermates are exclusively marine including the largest invertebrate “Giant Squids.”
- Symmetry usually radial, nearly always pentamerous.
- Body shape is rounded to cylindrical or star like.
- Surface of the body is rough.
- Body wall consists of an outer epidermis, a middle dermis and inner lining of peritoneum.
- Endoskeleton consists of closely fitted plates forming shell usually called “THECA,” may be composed of separate small “OSSICLES.”
- Coelom is spacious, lined by peritoneum and occupied mainly by digestive and reproductive systems.
- Presence of “Water Vascular System” is most characteristic feature.
- Alimentary tract is usually coiled.
- Circulatory or Haemal or blood lacunar system is typically present.
- Excretory system is wanting.
- Nervous system is primitive, consists of ganglionated nerve cord.
- Sense organs are poorly developed.
- Sexes are usually separate.
- Reproduction is usually sexual, fertilization is external.
Water canal system is unique in possessing an internal closed system of canals containing a watery fluid.
Regeneration of lost part is common
IMPORTANCE OF WATER CANAL SYSTEM
These canals are provided with tubular protrusions called “Tube Feet,” which serve a number of functions like locomotion, anchoring to hard surfaces, grabbing the prey, diverting food particles towards mouth and in some cases also respiration. The watery fluid is drawn from the surrounding water through a perforated disc called the “Madreporite.”
Star Fish, Brittle stars, Sea urchins, Sea-cucumbers, Sea-Dollar, Sea-lilies and Feather stars.