///. 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. 334. 



t IG. 325. 



FIG. 324. Oral area of criuoi&(Antedon\ showing by dotted lines the course of the in- 



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



arms to the mouth (orig.). 

 FIG. 325 Cross-section of pinnula of Antedon. (After Ludwig.) a, axial nerve 



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



canal ; t, tentacles. 



here have no suckers nor ampullae and are merely tactile tentacles. 

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

 hundred tubules leading from the ring canal to the ccelom. Oppo- 

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

 which water enters to pass through the tubules into the ambulacral 

 system. The ambulacral nervous system is weakly developed. 

 The enterocoale 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 problematical organ, 

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

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

 possibly a lymphoid gland, possibly a structure for the transfer of 

 nutriment, and is apparently homologous with the ' heart ' of the 

 starfish. 



