The Echinoid Directory

The sand-dollar test is reinforced by a series of buttresses, pillars or ridges that run between the upper and lower surfaces of the test. At their simplest there are just ten radially orientated wall-like partitions that are developed around the peripheral part of the test. This is the situation seen in most fibulariids. Clypeaster (interior of aboral surface illustrated below) has a more complex meshwork of concentric bars developed around the periphery and thin pillars centrally. But it is in the flattened sand dollars (scutellinids) that buttressing is best developed. In scutellinids much of the interior is filled with a maze of buttresses and pillars and only the spiral trace of the digestive system tract remains open. Buttressing acts to strengthen the test and allows sand dollars to have flattened thin tests for feeding in environments where wave pounding poses a potential danger.

Illustrations:

Upper. Cross-section through the test of the sand dollar Mellita.

Lower. Internal of aboral surface of Clypeaster, from Mortensen, T. 1948. A Monograph of the Echinoidea. IV(2). Clypeasteroida. C. A. Reitzel, Copenhagen.