The Echinoid Directory

One of the most striking features of many sand dollars is the presence of slit-like perforations that pass completely through the test. These perforations are termed lunules. Lunules can develop in one of two positions, along the midline of ambulacra, or along the midline of the posterior interambulacrum, between the peristome and periproct (the anal lunule). Ambulacral lunules may form either by resorption of the skeleton in the early stages of growth, or may begin as marginal indentations which become enclosed as growth proceeds. Anal lunules always form by resorption.

The function of lunules appears to be two-fold. Firstly they act as pressure drainage channels and thus help prevent the animal being lifted out of the sediment by wave action. Hydrodynamic tests have shown that sand dollars with lunules can live in settings with higher water flow rates.

Secondly they provide a coarse sift of the overlying sediment for its organic particles. Tube feet (tf) project into the lunule and capture larger particles that pass through the opening while ciliary currents direct finer particles to the food grooves (fg) that border the opening. Lunules are therefore also used to make food harvesting more efficient.