1.- Unique water-vascular system of coelomic origin extends from body surface as series of tentacle-like projections
(podia, or tube feet) protracted by increase of fluid pressure within them; opening to exterior (madreporite or
hydropore) usually present.
2.- Living in marine habitats.
3.- Free-living taxa.
4.- Body unsegmented (nonmetameric) with pentaradial symmetry; body rounded, cylindrical, or star-shaped, with five or more radiating areas, or ambulacra, alternating with interambulacral areas; no head.
5.- Triploblastic body.
6.- Coelom extensive, forming perivisceral cavity and cavity of water-vascular system; coelom of enterocoelous type;
coelomic fluid with amebocytes.
7.- Endoskeleton of dermal calcareous ossicles with spines or of calcareous spicules in dermis; covered by epidermis
(ciliated in most); pedicellariae (in some).
8.- Digestive system usually complete; axial or coiled; anus absent in ophiuroids.
9.- Skeletal elements connected by ligaments of mutable collagenous tissue under neural control, ligaments can be “locked” into rigid posture or relaxed to allow free movement at will; locomotion by tube feet, which project from ambulacral areas, by movement of spines, or by movement of arms, which project from central disc of body.
10.- Nervous system with circumoral ring and radial nerves; usually two or three systems of networks located at different levels in the body, varying in degree of development according to group.
11.- No brain; few specialized sensory organs; sensory system of tactile and chemoreceptors, podia, terminal tentacles,
photoreceptors, and statocysts.
12.- Autotomy and regeneration of lost parts conspicuous; asexual reproduction by fragmentation in some.
13.- Sexes separate (except a few hermaphroditic) with large gonads, single in holothuroids but multiple in most; simple
ducts, with no elaborate copulatory apparatus or secondary sexual structures; fertilization usually external; eggs
brooded in some; development through free-swimming, bilateral, larval stages (some with direct development);
metamorphosis to radial adult or subadult form; radial cleavage and regulative development.
14.- Excretory organs absent.
15.- Respiration by papulae, tube feet, respiratory tree (holothuroids), and bursae (ophiuroids).
16.- Blood-vascular system (hemal system) much reduced, playing little if any role in circulation, and surrounded by
extensions of coelom (perihemal sinuses); main circulation of body fluids (coelomic fluids) by peritoneal cilia.
Integrated principles of zoology / Cleveland P. Hickman, Jr. ... [et al.]. – 14th ed. p. cm.