///. CRINOIDEA. 



341 



ciliated and serve as conduits to bring food to the mouth. Nervous, 

 ambulacral, and blood systems begin with a circumoral ring. They 

 follow the ambulacral grooves as in the asteroids, but the ambulacra 



Fig. 324. 



Fig. 325. 



Pro. 321.— Oral areaof crii]oicl(.4ii(erfo)i), showing bj- dotted lines the course of the in- 

 testine from the mouth (m) to the anus ('/) ; ;y, ciliated grooves leading from the 

 arms to the mouth (orig.). 



Fig. 32.5 — Cross-section of pinnula of AulfOon. (After Ludwig.) a, axial nerve 

 cord; c, ciliated cups; c, c, coeliac canal; g, gonad; s, sacculi; .sc, subtentacular 

 canal ; (, teatRclcs. 



liere have no suckers nor ampulla and are merely tactile tentacles. 

 A typical stone canal is also lacking; in its place are five or several 

 liundred tubules leading from the ring canal to the coiloni. Oj)\m- 

 site their ccelomic mouths are fine pores in the oral disc through 

 which water enters to pass through the tubules into the ambulacral 

 system. Tlie ambulacra! nervous system is weakly develojjed. 

 The enteroccele system, on the other hand, is well developed and 

 forms the axial cord running through the brachialia and radialia 

 to unite in a ring in the centrodorsal. A pn-oblematical organ, 

 the so-called dorsal organ, also begins in the centrodorsal and 

 extends up through the axis of the tlieca to the oral disc. It is 

 possibly a lymphoid gland, j)0ssibly a structure for the transfer of 

 nutriment, and is aj^parently homologous with the ' heart ' of the 

 starfish. 



