The Echinoid Directory

A single spine is divisible into three parts, a long shaft, a short neck and a base. The concave articulation surface that attaches onto the mamelon of the tubercle is called the acetabulum, and this may be either perforate or imperforate. Muscles and catch apparatus connect from the base of the spine to the tubercle and allow each spine to be moved individually. In many regular echinoids the shaft is smooth and appears very finely striated under magnification. In a few groups, notably in arbacioids, the shaft has a glassy appearance and is covered in a thin, smooth cortex. In cidaroids there is a thick layer of cortex and the shaft is generally ornamented by ribs, thorns or granules. A very characteristic structure is seen in diadematoids, where the shaft is built up of whorls and appears segmented (the spine is then termed verticilate).

Spines are generally coloured, with pigment incorporated into the calcite of the skeleton.

In cross-section (SEM micrographs below) spines may be hollow or solid. The hollow core to a spine is called the lumen. Where the centre is filled with stereom, the unorganised central plug of stereom is termed the medulla.