A sea urchin spine
Sea Urchin Skeleton
Echinoderms (Phylum Echinodermata from Greek, ἐχῖνος, echinos – "hedgehog" und δέρμα, derma – "skin") are a phylum of marine animals. The adults are recognizable by their (usually five-point) radial symmetry, and include such well-known animals as starfish, sea urchins, sand dollars, and sea cucumbers. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes (a superphylum), after...