Crinoidea

Crinoidea is a small class of echin­o­derms with around 600 species. Many crinoids live in the deep sea, but oth­ers are com­mon on coral reefs.

In most ex­tant crinoids, pri­mar­ily the shal­low-wa­ter ones, there are two body re­gions, the calyx and the rays . The calyx is the cup-shaped cen­tral por­tion that lies below the oral sur­face, which is ori­ented away from the sub­strate; most of the or­gans are found in the calyx. Both the anus and the mouth open on the oral sur­face, but the anus is eas­ily dis­tin­guished by being lo­cated atop an os­si­fied cone set pe­riph­er­ally on the oral sur­face. The rays are long arm-like ex­ten­sions from the calyx that are used for feed­ing. Each ray has a lat­eral row of short branches on ei­ther side; these pin­nules in­crease the sur­face area and trap food. Some deep-sea crinoids have a third body por­tion, the stalk . It serves to an­chor the crinoid to the sub­strate. The stalk is largely com­prised of stacked cal­cite disks that are com­mon fos­sils in lime­stone. An­other con­spic­u­ous fea­ture of many crin­iods are long, thin pro­tru­sions called cirri . In un­stalked crinoids, the cirri are lo­cated on the end of the calyx op­po­site the mouth, and are used by the an­i­mal to grasp the sub­stra­tum. Cirri of stalked crinoids ex­tend from the stalk; they also seem to func­tion in ad­he­sion. The cal­citic os­si­cles of crinoids, as is typ­i­cal of echin­o­derms, form an in­ter­nal skele­ton that pro­vides sup­port and pro­tec­tion.

All crinoids are fil­ter feed­ers. The tube feet to move food par­ti­cles down the am­bu­lacral groove of a ray to­ward the mouth. Mod­i­fied os­si­cles called lap­pets that bor­der the am­bu­lacral groove func­tion to close off the groove and pre­vent dam­age to the tube feet.

The rays of crinoids are also im­por­tant for lo­co­mo­tion. By mov­ing their rays up and down through con­trac­tion and re­lax­ation of mus­cles, crinoids are able to swim slowly through the water.

A crinoid's in­ter­nal anatomy is dom­i­nated by or­gans for di­ges­tion and re­pro­duc­tion. The en­tire di­ges­tive sys­tem lies within the calyx and is char­ac­ter­ized by lit­tle more than a mouth and in­tes­tine with di­ver­tic­ula. The coelom ex­tends into the rays, where the go­nads are lo­cated. Nerves occur through­out the an­i­mal, but the mass found in the calyx seems to be the cen­ter for re­gen­er­a­tion of lost body parts. Ex­cre­tion may be ac­com­plished through small tubes called sac­cules lo­cated near the am­bu­lacral grooves, but the mech­a­nism for this is poorly un­der­stood.

Crinoids are gono­choric and brood their young until the em­bryo de­vel­ops into a do­lio­lar­ian larva or a fully formed ju­ve­nile crinoid.

All but one of the 9-11 sub­classes of crinoids are now ex­tinct and are known only through their some­times spec­tac­u­lar fos­sils. Ap­prox­i­mately 5,000 species of fos­sil crinoids are known, with the great­est di­ver­sity from the Pa­le­o­zoic. By the end of the Per­mian, how­ever, only one lin­eage seems to have sur­vived. The only sur­viv­ing sub­class of crinoids is the Ar­tic­u­lata.

Al­though crinoids are some­times amaz­ingly abun­dant, they ap­pear to have lit­tle com­mer­cial im­pact and hardly af­fect hu­mans in any way.

Ref­er­ences:

Hess, H., W.I. Au­sich, C.E. Brett, M.J. Simms. (1999) Fos­sil Crinoids. Cam­bridge Uni­ver­sity Press.

Kol­zoff, E. N. (1990) In­ver­te­brates. Saud­ers Col­lege Pub­lish­ing.

Mlade­nov, P.V., and Chia, F.S. (1983) De­vel­op­ment, set­tling be­hav­ior, meta­mor­pho­sis and pen­tacrinoid feed­ing and growth of the feather star Florome­tra ser­ratis­sima. Ma­rine Bi­ol­ogy 73:319-323.

Tasch, P. (1973) Pa­le­o­bi­ol­ogy of the In­ver­te­brates: Data Re­trieval from the Fos­sil Record. John Wiley & Sons, Inc.

Con­trib­u­tors

Derek Kel­logg (au­thor), Daphne G. Fautin (au­thor).

Glossary

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature