//. OPHIUROIDEA, 



337 



(p. 330), the ampullae as well as the five or more Polian and 

 Tiedemann's (racemose) vesicles projecting into the co3lom. 



Since the arms contain nearly all important organs, the physiological 

 independence of these is easily understood. Arms broken off not only 

 live, but regenerate first the disc and then 

 new arms which appear at first like small 

 buds (comet form, figs. 309, 310). This 

 separation of arms may occur through 

 accident, or it may be, and not infre- 

 quently is, produced by the animal itself. 



Examples of species with well-devel- 

 oped arms and ambulacra in four rows 

 are furnished by the ASTERID^E, repre- 

 sented on our shores by the five-finger 

 Aster ias * and Leptasterias,* and Heli- 

 aster* with numerous arms. In the 

 SOLASTERID^E the ambulacra are two- 

 rowed; arms sometimes numerous. Py- 

 tlionaster (fig. 316). In the ASTERINID^E 

 the arms are short or the body is pentag- 

 onal, no large plates on the margins of 

 the arms. Asteriscus (fig. 313). In other 

 forms (Culcita* fig. 317, Hippasteria* 



CtenodiSGUS*) the body is more Or less FIG. 



pentangular, the margin being covere.d 

 with large plates. 



Class II. Ophiuroidea (Brittle Stars). 



In these the animal consists, as in the Asteroidea, of disc and 

 arms, the latter sometimes branched, but the internal anatomy is 



different. The ambulacral plates 

 have been drawn inside the arm 

 and each pair fused to a large 

 'vertebra' (fig. 317). As a result 

 the co3lom of the arms is greatly 

 reduced, the hepatic caeca are lack- 

 ing, and the alimentary canal, which 

 lacks an anus, is confined to the 

 disc. By the ingrowth of ven- 

 FIG. SIT. Section of Ophiuroid arm tral plates the ambulacral grooves 



(ong.). a, ambulacrum; Z), blood ves- _ . G 



sei; c, coeiom; w, muscles of arm; n, are converted into tubes, and the 



nerve; ?, radial water tube; v/verte- , , , . , , _ . . 



bra ' (coalesced ambulacral plates). ambulacra, which lack SUCking 



discs, are tactile organs, locomotion being effected by the snake- 

 like motion of the arms. The madreporite is on the ventral sur- 



316 Pythonaster murrayf. 



