III. CRIXOID1.A 



301 



pinnules, lancet-shaped processes supported by calcareous bodies in which the 

 sexual products ripen until freed by dehiscence (fig. 298). 



The mouth opening, in the middle of the oral disc which closes the 

 theca, is frequently surrounded by five interradial plates, the oralia (fig. 

 205, B). The mouth, which in contrast to other echinoderms is directed 

 upwards, connects with a spiral digestive tract in which oesophagus, 

 stomach, and intestine can be distinguished. The anus is interradial 

 and near the mouth (fig. 297). Five ambulacral grooves begin at the 



A B 



FIG. 296. Hyocrinus bethleyainis. A, upper end of stalk with cup, and the bases 

 of the arms; b, basalia; br, brachialia; r, radialia. B, oral surface of cup with mouth, 

 five oralia, and the bases of the arms. 



mouth and extend out on the arms, branching with them and extending 

 to the tips of the pinnuke. In the ten-armed species (fig. 297) the grooves 

 fork near the mouth. These are ciliated and serve as conduits to bring 

 food to the oral opening. Nervous, ambulacral, and blood systems begin 

 with a circumoral ring. As in the asteroids, they follow the ambulacral 

 grooves to the pinnuke, but the ambulacra have no suckers nor ampullae 

 and are merely tactile tentacles. 



A typical stone canal is also lacking; in its place are five or several hundred 

 tubules leading from the ring canal to the ccelom. Opposite their coelomic 

 mouths are fine pores in the theca through which water enters to pass through 

 the tubules into the ambulacral system. The ambulacral nervous system is 

 weakly developed or may be absent. The apical system, on the other hand, 

 is well developed and forms the axial cord running through the brachialia and 



