Characters/ anatomy

Echinoids belong to the Phylum Echinodermata. Echinoderms are characterized by a water vascular system, internal mesodermal skeleton made up of calcite plates (normally spiny and covered outside and in by a thin protoplasmic skin), and pentameral (5-rayed) symmetry, though in some a secondary bilateral symmetry is imposed.

Regular echinoids live either on the sea floor or in cavities in rocks which they may bore themselves, while irregular echinoids have a burrowing mode of life. Terms which may be unfamiliar appear in bold throughout the text and are annotated on diagrams.


The common sea urchin, Echinus esculentus, illustrates well the fundamentals of anatomy in regular echinoids.

Diagram 1. Upper surface (left) and lower surface (right) of Echinus esculentus. From 'Basic Palaeontology' with permission of Professor M. J. Benton.


In common with most regular echinoids the calcareous skeleton, or test, is globular. If the spines are removed, there is a network of plates, divided into ten radial segments. Each segment is made up of a double column of interlocking plates, and segments extend from the upper (or aboral) to lower (or adoral) surface. The five narrower segments are the ambulacra. Ambulacral plates have sets of pore pairs near their outer edge. The five wider segments are the interambulacra. Interambulacral plates have tubercles to which spines articulate.

In the central part of the upper surface lies the apical disc: a double ring of larger genital plates alternating with smaller ocular plates. Each of the genital and ocular plates is perforated by a pore. One genital plate, the madreporite, is larger, with many tiny pores. The apical disc surrounds a central hole, the periproct, which contains the anus. On the lower surface the peristome surrounds the mouth. Both mouth and anus are located within a number of smaller plates on a membrane that extends across these holes, but is rarely fossilised. The test turns back at the edges of the peristome to form the perignathic girdle. The feeding apparatus, Aristotle's lantern, comprises five individual jaws, each with a single tooth, and operates as a 'grab', rasping at detritus and passing it inwards to the gut.


Most of the internal anatomy relates to the structures described above. In particular, the purpose of pores within the various plates becomes evident.

 Diagram 2. Section through an ambulacrum (left) and interambulacrum (right). From 'Invertebrate Palaeontology' with the permission of Professor E. N. K. Clarkson.

The water vascular system is a complex system of fluid-containing tubes and sacs. The only connection to the external sea-water is via the tiny pores of the madreporite in the apical disc. From this, the stone canal descends to unite with the circumoral ring. Five radial water vessels depart from this ring: one passes up the centre of each ambulacrum to terminate as a closed tube extending from an ocular pore. Branches from radial water vessels give rise to ampullae (sacs) and associated tube feet. Tube feet extend to the outside through the pore pairs of the ambulacral plates. The primary function of the water-vascular system is to operate the tube feet: in most regular echinoids these function in respiration and locomotion, and in adhesion to the substrate in some.

The digestive system lacks a true stomach. The gut is a simple tube which runs spirally round the inner wall from mouth to anus. The gonads are connected to the pores in the genital plates, from which gametes are released.


Regular echinoids can be defined as those in which the periproct (containing the anus) opens in the centre of the apical disc. In irregular echinoids the periproct lies outside the apical system, it has moved to either a posterior position or right down to the lower surface and a dominant bilateral symmetry is seen.

Irregular echinoids exhibit a range of adaptations to a burrowing life mode - from deep to shallow burrows, to those which live just below the surface. There have been various ways in which this burrowing strategy has evolved: all involve reshaping of the test and modification of the tube feet and spines in different ways to perform different functions. Two rather extreme examples are illustrated here: Echinocardium and the sand dollar Mellita.

Echinocardium lives in a deep burrow within the sediment, as seen in the picture to thr right.

On the upper surface, two pairs of ambulacra form slit-like 'petals' - the pores in these regions are enlarged and 'sticky' respiratory tube feet emerge from them to aid in respiration. The anterior ambulacrum ('ambulacrum' on the diagram) forms a groove around which surrounds the fasciole from which ciliated spines emerge in life to generate currents which pass down to the mouth. Enormous tube feet emerge from the anterior ambulacrum to create and maintain the burrow. Similar ones also emerge from the two posterior ambulcra in the lower regions to build a sanitary tube. On the lower surface one of the interambulacra forms an enlarged plastron: spines emerge from this area and may assist locomotion.

Diagram 3. (Top) Mode of life of Echinocardium. (Below) Upper (left) and lower (right) surfaces of Echinocardium. From 'Invertebrate Palaeontology' with the permission of Professor E. N. K. Clarkson.

Diagram 4: Upper (left) and lower (right) surfaces of Mellita. From 'Invertebrate Palaeontology' with the permission of Professor E. N. K. Clarkson

The sand dollar Mellita lives either on the surface or buried horizontally within the sand. It is highly flattened.

Petaloid ambulacra are also seen here. There also exist perforations or holes in the test - the lumules, five of which are seen here. Four lunules are in the ambulacra and thought to increase perimeter of test for food gathering. On the lower surface five channels or food groves are lined with tube feet.

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