VIII 



EGHINODERMATA 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 

 Jfi/ocrinus (Fig. 298), the extant unstalked 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 8- 

 larva of Antedon and with 

 ffaplocrinus, 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 Pl - 20S. Hyocrinus Bethellianus (after P. H. 

 rnlvv milv a vprv small npri Carpenter). Tegmen calycis. 1, Axial canal of the 



caiyx only a very small pen- brachials . 2> extension of body cavity in the anu . 



pheral Zone remains. In 3, food groove of the arm ; 4, smaller plates of the 



HlfOCrimiS (Fia;. 298) also, and tegmen; 5, orals; 6, anal cone; 7, oral edges of the 



radials 



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 Rhizocrinus 

 Rawsoni they are almost rudi- 

 mentary, so that the zone 

 which surrounds them forms 

 the greater part of the 

 tegmen. 



In the Cyathocrinidce (Tn- 

 adunata Jistulata), 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. ^^ wh(jn they ^ ^ 



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

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



