Anus

madreporite madreporite

nerve ring mouth gonopore gonad axial gland ampullae esophagus siphon intestine test radial canal ring canal Polian vesicle Aristotle's lantern pharynx radial nerve peristomial membrane nerve ring mouth

Sea urchin and sand dollar anatomy. (Illustration by Emily Damstra) Grzimek's Animal Life Encyclopedia

A toxic sea urchin (Tripneustes gratilla) covers itself with debris as camouflage. (Photo by ©A. Flowers & L. Newman/Photo Researchers, Inc. Reproduced by permission.)

Sand dollars burrow into the sediment by moving at an angle to the surface, and heart urchins burrow by excavating allowing the animal to sink vertically into the sediment. Forward movement is then achieved by using its powerful laterally orientated spines.

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