vni ECHINODERMATA 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 (?). 



The same condition is found in the extant genera Holopus and 

 Hyocrinus (Fig. 298), the extant tmstalked genus Thaumatocrinus, 

 and the extant canaliculate 

 genus ' Rhizocrinus. All these 

 genera possess five oral plates, 

 which, however, are separate, 

 and do not form a closed 

 pyramid ; the mouth, there- 

 fore, is in open communica- 

 tion with the exterior between 

 them. Compared with the 

 larva of Antedon and with 

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



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 

 Rhizocrinus lofotensis the orals 

 are smaller, and in Rliizocrinus 

 Rawsoni they are almost rudi- 

 mentary, so that the zone 

 which surrounds them forms 

 the greater part of the 

 tegmen. 



In the Cyathocrinidw (In- 

 adunata fatulata), five large 

 plates can sometimes be dis- 

 tinctly made out in the centre 

 of the plated tegmen ; some- 

 FIG. 299. -system of plates of the tegmen of times, however, irregular 



Platycrinus tuberosus (after Wachsmuth and pieces are found in their 

 Springer). For lettering see p. 317. placeg> Wh(m they are dig . 



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

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



FIG. 298. 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 



