Echinodermata

Echinos→ spine

derma → skin

Jacob Klein introduced the term Echinodermatain 1734.

General characteristic feature of phylum Echinodermata:

• They are exclusively marine, not found in Nepal.
• The body possesses numerous spines, five radiating ambulatory groups used for protection to the body.
• They are triploblastic and radial symmetrical ( larval forms are bilaterally symmetrical.)
• They are coelomate animals. Coelom is the enterocoelous type.
• The body is of various shapes like globular, star-shaped, spherical, elongated and flattened. The distinct head is absent. The body has distinct oral and aboral surfaces. The body surface is marked by five radiating areas called ambulacral areas.
• A characteristic feature of echinoderms is the water vascular system. This system helps the animal in feeding, locomotion. It takes place by tube feet. It is present on each arm.
• The alimentary canal is complete which is coiled or straight. Mouth is present on the oral surface and anus on aboral surface.
• Respiration takes place through gills or cloacal respiratory trees.
• Blood vascular system is enclosed in coelomic peri haem channels. Respiratory pigments are absent.
• The excretory organ is absent.
• The nervous system is without the brain but a circumoral ring and radial nerves are present. Hence, it is of the primitive type.
• Sexes are separated. Fertilisation is external. They have the remarkable power of regeneration.
• It is indirect through free-swimming larval forms such as bipinnaria and auricularia.

Classification:

Phylum Echinodermata is classified on the basis of number and type of arm and form of the body.

Subphylum-1 Eleutherozoa:

There are the free swimming stemless echinoderms. It has five classes:

Class-1: Asteroidea

• The body is star-shaped with five radiating arms, which are not sharply marked off from the central disc.
• Ambulacral grooves with tube feet.
• Oral and aboral surfaces are distinct.
• Example: Asterias

Class-2: Ophiuroidea

• The body is star-shaped having five arms, which are sharply marked off from the central disc.
• Ambulacral grooves absent or covered with ossicles.
• The anus is absent.
• Example: Ophioderma

Class-3: Echinoidea

• The body is spherical or disc-like oval.
• Arms are absent.
• Ambulacral grooves covered with ossicles.
• Aristotle's lantern is a masticatory apparatus present.
• Example: Echinus

Class-4: Holothuroidea:

• The body is elongated without arm and spine.
• Mouth surrounded by tentacles.
• Longitudinalambulacral grooves and scattered ossicles.
• Example: Holothuria

Subphylum:2 Pelmatozoa ( attached to substratum by stalk)

Class-5: Crinoidea

• Usually stalked animal with more or less globular body.
• Many arms with pinnules forming feathers.
• Ambulacral grooves and scattered ossicles.
• Mouth and anus are present on the oral side.
• Example: Antedon