VIII EGHINODEBMATA— MORPHOLOGY OF SKELETON 



335 



which the food-grooves pass out on to the arms. The posterior oral 

 plate is somewhat larger than the others, and has a perforation 

 which may be the anus (1). 



The same condition is found in the extant genera Holopus and 

 Hi/ocrinus (Fig. 298), the extant unstalked genus Thaumatociiims, 

 and the extant canaliculate 

 genus Rhi-ucrinus. All these 

 genera possess five oral plates, 

 which, however, are separate, 

 and do not form a closed 

 pyramid ; the niouth, there- 

 fore, is in open communica- 

 tion with the exterior between 

 them. Compared with the 

 larva of Antedon and with 

 Htqilocrinus, Holopus shows 

 the most primitive (or em- 

 bryonic) condition, since in 

 it the oral pyramid is large, 

 covering nearly the whole of 

 the tegmen, so that between 

 its base and the edge of the 

 calyx only a very small peri- 

 pheral zone remains. In 

 Hyocrinus (Fig. 298) also, and 

 Thaumatocrinus the orals are 

 still of considerable size, but the peripheral zone, which is beset with 

 small closely -crowded plates, is somewhat broader than in Holopus 



(about one-fifth the diameter 

 of the whole tegmen). In 

 Bhizocrinus lofotensis the orals 

 are smaller, and in Bhizocrinus 

 cjoct/ Bawsoni they are almost rudi- 

 mentary, so that the zone 

 which surrounds them forms 

 the greater part of the 

 tegmen. 



In the Gyathocrinidce {In- 

 adunata fistulata), five large 

 plates can sometimes be dis- 

 tinctly made out in the centre 

 of the plated tegmen ; some- 

 -System of plates of the tegmen of times, however, 



Fig. 308. —Hyocrinus Bethellianus (after P. H. 

 Carpenter). Tegmen calycis. 1, Axial canal of the 

 brachials ; 2, extension of body cavity in the arm ; 

 3, food groove of the arm ; 4, smaller plates of the 

 tegmen ; 5, orals ; 6, anal cone ; 7, oral edges of the 

 radials. 



Fig. 



Platyorlnus tuberosus (after Waclismutli and pieces 

 Springer). For lettering see p. 317. 



irregular 



are found in their 

 places. When they are dis- 

 tinct, the posterior plate is the largest, and is. sometimes shifted 

 anteriorly between the others. In all cases they cover the mouth in 



