Radially symmetrical. Have dermal skeleton which consists of

calcareous ossicles that may develop into spines.

With water vascular system which connect with exterior by a pore(madreporite).

With tube feet which mainly serve locomotion but may alsoo assist in sensation,respiration and food collection.

Star shaped. The mouth opens on lower side(oral

surface)and anus,if present,on the upper side(aboral surface)

The arms are not sharply marked off from the central disc.

Each arm has an open (ambulacral groove)which contains raws of tube feet ending with suckers.

Pedicellari present. Madreporite present on aboral surface. Ex. Asteropecten sp. (Star fish)

A series of tube feet project in 2 raws along each ambulacral groove,the terminal tube foot or tentacle on the tip of each arm has a pigmented spot(the eye) at its base(light sesitive).

The skin is covered with calcareous plates(ossicles) and on these plates arise spines of different sizes.

Very small spines(pedicellaria)serv eto clean the body surface

On the aboral surface,near the periphery of the disc there is the madreporite.

Star shaped with arms sharply marked off from the central disc.

The ambulacral grooves are closed and covered by ossicles.

Tube feet without suckers. No pedicellaria. Madreporite on the oral surface,no

anus. Ex. Ophicoma sp. (Brittle star)

On the oral surface,there is the mouth opening surrounded by 5 buccal plates,one of them is modified into madreporite.

The arms are surrounded with 4 longtudinal raws of skeletal plates and 2 raws of tube feet project between them(sensory and respiratory).

With compact skeleton without arms. Ambulacral grooves covered with

ossicles. Tube feet ending with suckers. Anus and madreporite are aboral. Pedicellaria well developed. Ex. Tripneustes sp. (Sea urchin)

Body is divided into 10 areas between the 2 poles,5 ambulacral areas containing the tube feet and the other 5 are the inter ambulacral areas with spines and with out tube feet.

On the oral pole,there is the mouth surrounded by 5 pairs of sensory buccal tube feet in a circle.

There are also 5 pairs of branchiae help in gas exchange.

At the aboral pole,there is a central circular membrane known as the periproct through which the anus open eccentrically.

The skeleton consists of large no. of ossicles fused together known as the corona or shell below the outer skin.

With elongated body,cucumber or worm-like shape,without arms.

Usually without madreporite ,mouth and anus lie at opposite ends of the body.

Muscular body wall with no spines and only small ossicles.

Ambulacral grooves are covered,some of the tube feet are modified into tentacles around the mouth,some of them are with suckers.

No pedicellaria Ex. Holothuria sp. (Sea cucumber).

Sessile echinoderms attached by the aboral surfaces.

Oral surface directed to the upper side containing mouth and anus.

Tube feet with out suckers,ambulacral grooves are open.

No madreporite,no spines,nopedicellaria.

Ex.Heterometra sp.(Sea feathers)

The body is compossed of small central disc or calyx surrounded by 5 arms each bifurcates twice,thus are formed of 20 arms each fringed with double series of short branches khnown as pinnules giving it the feathery appearance.

In each arm there is an open ambulacral groove surrounded by tube feet help with pinnules in pushing food particles to mouth.

The aboral surface with appendages called cirri by which the animal clings to the ground.