The Spiny - Skinned Animals:
From arms length a common sea star appears to have a smooth surface with some white beads. The beads are actually short spines, made mostly from calcium carbonate, a compound extracted from sea water. A dried sea star with skin removed shows a meshwork of bony plates and spines. This is their endoskeleton - calcareous ossicles (bone-like fragments) embedded in connective tissue. Many of these ossicles support outward directed spines (like these pictured here). These spines are controlled by muscles and are used as protection from predators and sometimes aid in locomotion. If you look between the spines, you may find thin, non-calcified appendages called papulae that protrude between ossicles and aid in respiration. Pictured here is a close-up of the spines and skin of a rose star.