sand dollar

Echinarachnius parma Any echinoderm of the order Clypeasteroida, having a flat, disk-shaped body with the mouth in a mid-ventral position, and living in sand, on or near the surface.
1987, John M. Lawrence, A Functional Biology of Echinoderms, http://books.google.com.au/books?id=70oWAQAAIAAJ&q=%22sand+dollars%22+echinoderms&dq=%22sand+dollars%22+echinoderms&hl=en&sa=X&ei=vXaAUr3VMYeVkAWkiICoBQ&ved=0CIgBEOgBMBM page 75,

In the sand dollars, the arrangement of the food grooves and consequently the lateral water vessels and their accessory tube feet is more complex (Phelan 1977).

Adult echinoderms have a five-part body plan corresponding to the arms of a sea star or the design on the "shell" of a sand dollar.

If you look carefully, a sand dollar is merely a sea urchin in flattened form.