Starfish are the best known echinoderm group. Echinoderms are dioecious and fertilization is usually external. The nervous system consists of two central nerve rings with radial nerves to the periphery. The hemal system may be a transport system that delivers nutrients from the gut to the coelomic compartments for local distribution. A hemal system is present but its role in transport is still poorly understood and the chief transport system is the circulating fluid of the various coelomic compartments. A variety of gas exchange structures, including the tube feet, is found in various echinoderms. The connective tissue is changeable and its consistency is under nervous control.Įxcretion in echinoderms is by simple diffusion of metabolic wastes (ammonia) across thin permeable regions of the body wall. In most echinoderms, calcareous spines of various sizes and shapes arise from the dermis and extend from the body surface hence the name echinoderm (= ‘spiny skin’). These ossicles make up an endoskeleton which assumes different forms in different taxa. The body wall includes a thick connective tissue dermis with calcareous ossicles (‘little bones’). The water vascular system is an important echinoderm characteristic that in most groups controls the locomotory tube feet, but is also important in gas exchange, excretion, and feeding. About 6000 extant species are known, but the fossil record includes about 13,000 extinct species. The adult radial symmetry is pentamerous, with body parts occurring in fives or multiples of five. ![]() Modified, with permission, from Invertebrate Anatomy OnLineĬopyright 2003 by Richard Fox (Lander University)Įchinoderms are secondarily radially symmetric - their ancestors were bilaterally symmetric.
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