General features Size range and diversity of structure Although most echinoderms are of small size, ranging up to 10 centimetres four inches in length or diameter, some reach relatively large sizes; e. Among the largest echinoderms were some extinct fossil crinoids sea lilies , whose stems exceeded 20 metres in length. Although all living echinoderms have a pentamerous five-part radial symmetry , an internal skeleton , and a water-vascular system derived from the coelom central cavity , their general appearance ranges from that of the stemmed, flowerlike sea lilies, to the wormlike, burrowing sea cucumbers, to the heavily armoured intertidal starfish or sea urchin. The general shape of the echinoderm may be that of a star with arms extended from a central disk or with branched and feathery arms extended from a body often attached to a stalk, or it may be round to cylindrical.
Starfish - Wikipedia
Feeding[ edit ] Close-up on the pinnules with visible rows of translucent podia. Crinoids feed by filtering small particles of food from the sea water with their feather-like arms. The tube feet are covered with a sticky mucus that traps any food that floats past. Once they have caught a particle of food, the tube feet can flick it into the ambulacral groove, where the cilia are able to propel the stream of mucus towards the mouth.
Pedicellaria and papulae of Asterias forbesi The body wall consists of a thin cuticle, an epidermis consisting of a single layer of cells, a thick dermis formed of connective tissue and a thin coelomic myoepithelial layer, which provides the longitudinal and circular musculature. The dermis contains an endoskeleton of calcium carbonate components known as ossicles. These are honeycombed structures composed of calcite microcrystals arranged in a lattice. They remove debris from the body surface and wave around on flexible stalks in response to physical or chemical stimuli while continually making biting movements.
Their bodies can become stone hard or in holothurians it can become so soft that it may flow between fingers. If attacked by a predator, brittle stars can break their arms at will and grow them again later. This is called Autotomy. Brittle stars do not have intestine, anus, dermal branchiae, pedicellariae and ambulacral grooves.