The sea star skeleton is made of small platelets or ossicles of magnesium-rich calcium carbonate. It's an endoskeleton, lying under a surface of skin tissue, that is usually so transparent it is difficult to detect. The brilliant colors of sea stars are a result of pigments in the skin and surface of the ossicles. In this species, Culcita novanugineae, short purple spines contrast with brilliant yellow colors on the flat ossicles.

The matrix of ossicles on a sea star surface is riddled with holes that can be easily seen in this view. These are passage ways for thin finger-like projections -the skin gills. You can see the skin gills sticking out as a kind of fuzzy blur on the left of the photograph.

In most sea stars, the flat ossicles can move in relation to one another, so the animal can bend its arms. If turned upside down, a sea star can right itself by flexing its arms - rotating the tips of one or more arms so that the tube feet can grasp the sea floor.