The Echinoid Directory

Regular sea urchins have a pentaradially symmetrical test composed of ten double columns of plates, five interambulacral zones and five ambulacral zones, which together make up the corona. Ambulacral zones are made of plates that are pierced by single or double pores for the tube-feet. These pore-pairs are situated along the outer (adambulacral) margins of the ambulacral zone. The central suture line between the two columns of plates in each ambulacral zone is the perradial suture line. The interambulacral plates abut the ambulacral zones along the adambulacral suture, and the midline between the two columns of interambulacral plates is the interradial suture

There are two major openings in the test, the peristome and periproct. The peristome is circular and central on the lower surface and it is here that the mouth opens. In life much of it is covered in a thick skin with embedded plates (the imbricate embedded plates are still in position in the specimen illustrated). All ambulacral and interambulacral zones converge around the mouth.

At the apex of the test, at the point of origin of the ambulacral zones, lies the apical disc. This ring of ten plates encircles the anal opening and is always positioned on the aboral surface. The apical disc is composed of five genital and five ocular plates which surround a flexible plated membrane that contains the anus. This flexible membrane is termed the periproct. The arrangement of plates in the apical disc provides important taxonomic characters.

The precise arrangement of ambulacral plates on the test is highly variable and is extremely important. More information on this character is given under ambulacral plating.

All plates bear spines, which attach and articulate at tubercles. In the specimen illustrated there is a single large primary tubercle on each interambulacral plate, and numerous small tubercles and granules along the outer margins of interambulacral plates and the perradial zone of ambulacral plates. Tubercles can be perforate or imperforate and crenulate or smooth: more information on tubercle structure is available. The spines act both for defence and locomotion and come in a variety of forms (see spines).

Inside the test is a complex apparatus termed the Aristotle's lantern. This acts as the animal's jaws, and comprises some 50 skeletal elements and 60 muscles. The morphology of the lantern is very important for the higher taxonomy of this group.

The only other internal structure of importance is the perignathic girdle. This is a structure formed from the plates immediately surrounding the peristome, and forms the site of attachment for the muscles that work the Aristotle's lantern. Again the detailed arrangement of how this is constructed provides useful taxonomic characters.