Starfish
A starfish's body wall consists of a thin, outer epidermis, a thick dermis formed of connective tissue, and a thin, inner peritoneum, which contains longitudinal and circular muscles. The dermis contains rather loosely organized ossicles (bony plates). Some bear external granules, tubercles, and spines, sometimes organized in definite patterns and some specialized as pedicellariae. There may also be papula, thin-walled protrusions of the body cavity that reach through the body wall and extend into the surrounding water, which serve a respiratory function. These structures are supported by collagen fibers set at right angles to each other and arranged in a three-dimensional web with the ossicles and papula in the interstices. This arrangement enables both easy flexion of the arms by the starfish with the rapid onset of stiffness and rigidity required for actions performed under stress.