Sea Urchins and Sand Dollars The Disc and Globe Echinoderms

The sea urchins and the sand dollars together comprise a group of echinoderms known as the Echinoidea (or more commonly as the echinoids). These echinoderms are globe or disc shaped and are either radially symmetrical or in some cases bilaterally symmetrical.

There are about 940 species of echinoids that are sorted into two subgroups. The smaller subgroup, the Perischoechinoidea, are thought to be the more primitive of the two groups. There are about 140 living species in this group and most have large, pencil-like spines. The other subgroup of echinoids are the Euchinoidea, a group consisting of about 800 species.

Sea urchins and sand dollars, like all echiniderms, have ossicles, tiny particles of calcium carbonate that form an endoskeleton. But in sea urchins and sand dollars, the ossicles are fused together to form a test (it is this structure that you might be familiar with - the test of sea urchins and sand dollars are often seaside souvenirs).

Sea urchins also have a uniques feeding structure called Aristotle's lantern. This structure is located at the center of the body and includes five teeth that when moved together form a complex grinding tool. This enables them to efficiently tear apart food. In sand dollars, the structure is somewhat modified and the teeth are smaller. Some echinoids lack the Aristotle's lanter entirely.