Echinoderms are organisms characterized by having an internal skeleton of porous calcite plates, which are normally spiny and covered outside and inside by a thin protoplasmic skin.

Echinos = spiny derma = skin

Normally the skeletons have a five - rayed or pentameral symmetry, though in some fossil groups and some modern sea - urchins there is a bilateral symmetry.

Echinoderms are exclusively marine animals, they appeared in Cambrian and are still living. Representatives today include starfish and sea - urchins which are so common in shallow waters.

Because of the calcitic nature of echinoderm skeleton; they are very abundant in the fossil record and often their remains have greatly contributed to carbonate sediments.

Ex. Crinoidal limestone

This is an internal apparatus in Echinodermata. It consists of five radial canals which are connected with tube feet.

The water vascular system is used to operate the tube feet by creating water pressure in the tube feet, by this way they can extend outside the skeleton (test). The system gets its water from the canals which are joined with the ocular pores at the upper part and from the madreporite.

Tube feet are tubes connected with the water vascular system. They extend outside the test through pores found in the skeleton. They are used for locomotion, respiration and feeding.

1- Subphylum: Pelmatozoa ------------> Sessile

2- Subphylum: Eleutherozoa -----------> Free - moving

1- Class: Echinoidea ----------> Ex. sea urchin

2- Class: Stelleroidea ----------> Ex. star fish.

3- Class: Holothuroidea ------> Ex. sea cucumber

Class Echinoidea is the most important as fossils and will be studied here in detail.

Echinoids have a test consists of four parts:

* Corona.

* Oculo- genital system (apical system).

* Peristome (around mouth).

* Periproct (around anus).

The corona consists of ten segments extending from the apical disc on the upper surface to the peristome on the lower surface.

The ambulacra and interambulacra consist of calcite plates which meet along a zigzag suture.

The ambulacral plates have pores arranged in pairs near the outer edge of the plate. There are no pores in the interambulacral plates but are tubercular.

They are the sites where the tube feet emerge through the test from the internal part of the water vascular system.

The apical system is a disc of 10 plates surrounding a central hole (Periproct) which contains the anus. Five plates of these are larger called genital plates and are perforated by pores called genital pores through which eggs and sperms go outside. The other five plates are smaller and called ocular plates. They are also perforated by ocular pores, which are connected by tubes of the water vascular system.

One genital plate is larger than the others, this is called madreporite. It has numerous tiny perforations, which lead into the water vascular system below.

This is a central hole containing the anus on the upper surface (aboral surface).

Aboral ----------> away from the mouth

This is a sheath surrounding the mouth and Aristotle's lantern. It lies on the lower surface (adoral surface).

Adoral ---------> towards the mouth

This is a lantern-shaped structure located on the lower surface of echinoid test.

It has five strong jaws, each with a single calcitic tooth. The jaws are used to catch organic material on the sea floor and pass it inwards to the mouth.

There are two kinds of echinoid tests; regular and irregular:

These forms have pentameral symmetry. They live either on the sea floor or in cavities and can move easily in any direction.

These forms are usually heart - shaped and have a bilateral symmetry.

They are usually infaunal and can move in one direction.

This subphylum includes all attached (sessile) echinoderms. It is divided into four classes:

1- Class: Cystoidea (Cambrian - Permian).

2- Class: Blastoidea (Silurian - Permian).

3- Class: Edrioasteroidea (L. Cambrian - Carboniferous)

4- Class: Crinoidea (M. Cambrian - recent).

Note that: sessile echinoderms dominated during Paleozoic, most of them are extinct except Crinoids.

Crinoids are echinoderms, which live in congregates called lillie gardens and include sea lillies or stone lillies (fossils).

The skeleton consists of three parts:

The stalk consists of a number of segments called columnals, these columnals are articulating together to give the stalk a sort of flexibility for resisting waves.

The columnals are pierced centrally for the passage of blood vessels and nerves.

This is the part of skeleton in which the organism lives. It is cup - shaped and may be covered with a leathery covering termed tegmen.

The calyx consists of a number of plates arranged in cycles about a central plate called Centrodorsal plate, located at the top of the stem and forming the base of the calyx. The plates from base to top of calyx are called infrabasals, basals, radials, and interradials.

Crinoids with infrabasals are called Dicyclic, some crinoids don't have infrabasal plates (monocyclic).

These are usually five in number, each arm consists of a single row of plates tapering to the free end.

Some crinoids are attached to the substrate by their stem (stalk ), others have roots.

Crinoids are still living today and were also abundant in the Paleozoic