The echinoderms include starfishes, brittle-stars, sea-urchins, sea-cucumbers, feather-stars and sea-lilies and are one of the most distinct and well-known groups of shore animals. All are marine, rarely being recorded from brackish water, and are found from the seashore to the deepest waters. The surface of the body is often brightly coloured and is generally spiny or warty. Echinoderms are characterized by a unique five-rayed or pentamerous radial symmetry very clearly displayed in the starfishes and brittle-stars and usually readily detected in the globular sea-urchins and the elongate sea-cucumbers. Calcareous plates are present in the body wall. In starfishes these are held together by connective tissue rendering the body wall flexible; in sea-urchins the plates are fused to give a rigid test, and in sea-cucumbers they are isolated within the tissue. In all echinoderms, a delicate epidermis overlies the skeletal plates and the skeleton is therefore an endoskeleton. Spines are characteristic of the phylum, and in asteroids and echinoids specialized structures known as pedicellariae (see Figs. 18.7, 18.12) are also found on the body surface. These are pincer-like in appearance, sometimes on long stalks, and equipped with adductor muscles enabling the jaws to be snapped shut. Some are equipped with poisonous glands. They are used both in defence and as a protection against smothering by particles of detritus and small organisms such as settling larvae. They are important in identification.