CRINOIDEA. 



259 



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," and (4^ the stalk supporting 



-a. 



FIG. 122. Diagrammatic vertical section through disc and 

 base of one of the arms of Antedon rosacea. After 

 M lines Marshall. 



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

 ated openings in body wall ; /z., subepithelial ambulacral nerve ; 

 /. , water-vascular cnnal ; A., tentacle ; r., mouth ; s., intestine ; 

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

 ccelom ; R^.-R'^., radial plates ; Br., brachial plates ; ., muscle ; 

 a., axial nerve-cord; d., central capsule; C.D., centro-dorsal 

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



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

 "centro-dorsal" ossicle, bearing the cirri; this conceals the coalesced 

 " basals" of the larva ; above this are three tiers of " radials," whence 

 spring the "brachials" of the arms. 



The oral disc, turned upwards, is supported by plates. Here the 

 anus also is situated. The arms usually branch in dichotomous fashion, 

 and thus ten, twenty, or more may arise from the original five. But the 

 growing point continues to fork dichotomously, like the leaf of many 

 ferns, and as each alternate fork remains short, a double series of lateral 

 " pinnules " results. The arms are supported by calcareous plates. The 

 stalk usually consists of numerous joints, especially in extinct forms, in 

 some of which it measured over fifty feet in length. Except in Holopus, 

 and in the stalked stage of Antedon, the stalk bears lateral cirri. 



