The Echinoid Directory


Tubercles are the attachment point for spines on the test. They come in many shapes but all are constructed along the same basic plan. There is a ball-like mamelon (m) at the centre, which is the articulation surface for the spine. This is smooth and shiny because it has a solid calcite veneer. It may either be perforate (as in the photograph above) or imperforate (below right), the perforation housing a ligament that runs from the plate to the spine. The mamelon is surrounded by a narrow parapet which is smooth in the specimen illustrated above, but is crenulate in the specimen photographed lower left. The crenulations match similar crenulations on the base of the spine and help lock the spine in place. Whether tubercles are perforate or imperforate and crenulate or non-crenulate, are important taxonomic characters. The mamelon is situated at the apex of a conical mound, the boss (b). This is the site of connective tissue attachment (used for locking the spine in a defensive position). Finally, the outermost zone, which is often slightly depressed defines the areole (ar). This is where the muscle that is used to move the spine attaches.

There may be one primary tubercle to a plate, or several. There are also normally a variety of smaller tubercles and granules. In the specimen above there is a distinct ring of secondary tubercles surrounding the areole of the primary tubercle. These are the scrobicular tubercles, and they form a scrobicular ring around the primary tubercle.

The tubercles illustrated below are from a diadematoid (crenulate and perforate) and a camarodont (non-crenulate and imperforate); the latter is an SEM micrograph.