CRINOIDEA. 



273 



'water, and many in the great abysses. An anchorage is 

 found on rocks and stones, or in the soft mud, and great 

 numbers grow together a bed of sea-lilies. The free 

 Comatulids swim gracefully by bending and straightening 

 their arms, and they have grappling " cirri " on the aboral 

 side, where the relinquished stalk was attached. By these 

 cirri they moor themselves temporarily. Small organisms 

 Diatoms, Protozoa, minute Crustaceans are wafted down 

 ciliated grooves on the arms to the central mouth, which 

 is of course on the upturned surface. Some members of 



FIG. 140. Diagrammatic vertical section through disc and 

 base of one of the arms of Antedon rosacea. After 

 Milnes Marshall. 



The section is inter-radial on the left, radial on the right. , Cili- 

 ated openings in body wall ; A., sub-epithelial ambulacral nerve ; 

 /., water-vascular canal; k., tentacle; r. t mouth; s., intestine; 

 -., central plexus, with "chambered organ" at its base ; /., 

 ccelom ; 7? 1 .-/? 3 ., radial plates ; J3r., brachial plates ; ., muscle ; 

 a., axial nerve-cord; d. y central capsule; C.Z>., centro-dorsal 

 plate ; /., cirri ; *., nerve branches from central capsule to cirri. 



the class, e.g. Comatula^ are infested by minute parasitic 

 " worms " (Myzostomata) allied to Chaetopods, which form 

 galls on the arms. A lost arm can be replaced, and even 

 the visceral mass may be regenerated completely within a 

 few weeks after it has been lost. It has been suggested that 

 the occasional expulsion of the visceral sac frees the Crinoid 

 from parasites (Dendy). 



The animal consists of (i) a cup or calyx, (2) an oral disc forming the 

 lid of this cup, (3) the radiating "arms," arid (4) the stalk supporting 

 the whole. The lowest part of the cup is supported by a pentagonal 



