The Echinoid Directory

Skeletal morphology of heart urchins and their relatives

Heart urchins (spatangoids) and their relatives (holasteroids, disasteroids) have a bilaterally symmetrical test composed of ten double columns of plates, five interambulacral columns (labelled 1-5) and five ambulacral columns (labelled I-V). Ambulacral plates are pierced by single or double pores for the tube-feet. The anterior ambulacrum is ambulacrum III. In this specimen the individual plates that make up the test are readily visible; the plate boundaries are light and the centres dark.

There are two major openings in the test, the peristome and periproct. The oval to D-shaped opening on the lower surface is the peristome, and this is where the mouth is situated. All five ambulacra converge onto the peristome, and it is almost always positioned close to the anterior border. The periproct houses the anal opening and is always positioned towards the posterior. Depending upon the species, it may open slightly above the ambitus so as to be visible from above, open on a vertical truncate face, or it may open subambitally and be visible in oral view. It is always surrounded by plates of interambulacrum 5.

At the apex of the test, at the point of origin of the ambulacral zones, lies the apical disc. This is composed of a small number of plates that are the first to form at metamorphosis. It is made up of between one and four genital plates, pierced by gonopores, which connect the gonads to the exterior, and five ocular plates, at the growing tip of each ambulacrum. The plating arrangement of the apical disc provides a number of important taxonomic characters.

The adapical portions of ambulacra are variously developed into petals, specialised zones of enlarged pore-pairs that support respiratory tube-feet. The shape and relative development of petals is important taxonomically. In some taxa all five ambulacra are identical. Spatangoids typically have sunken petals, while other groups have all ambulacra flush on the surface. Often the anterior ambulacrum (ambulacrum III) is differentiated from the others, as in the taxon illustrated. The degree to which the frontal ambulacrum is differentiated is important: it varies from being flush to deeply sunken, and the development of pore-pairs in this ambulacrum can also vary considerably. Tube-feet in this region of the test are responsible for constructing a respiratory tunnel in burrowing species. Enlarged pore-pairs may also be present on ambulacral plates at the rear of the test for constructing a sanitary tunnel.

On the oral surface, the plating of the posterior interambulacrum (behind the peristome) is extremely important taxonomically. This zone is termed the plastron, and is often the most densely tuberculated part of the test.

Spines and tubercles are generally rather fine in heart urchins, although some taxa have scattered large primary tubercles over their aboral surface which support long, sharp spines. Aboral tubercles are always finer and more dense than oral tubercles. Tubercles are always perforate and typically crenulate, although a great deal of specialisation in both spine and tubercle structure is found in this group. Spines are long and slender, and often have spatulate tips. Burrowing heart urchins have bands of very fine spines that are termed fascioles that help them live in fine sediments. The detailed shape and position of these fascioles is important taxonomically.

Heart urchins are deposit feeders using their oral tube-feet to gather detritus. They have no lantern internally (even during their early ontogeny), and no perignathic girdle.