III. CRINOIDEA 



301 



pinmdcc, 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 live 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 (lig. 297). Five ambulacra! grooves begin at the 



A B 



Fig. 296. — Tlyocrinus belhleyaniis. A, upper end of stalk with cup, and the bases 

 nf tlie arms; b, basalia; br, Ijrachialia; r, radialia. B, oral surface of cup with mouth, 

 live oralia, and the bases of the arms. 



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

 to the tips of the pinnultc. 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, ambulacra!, and blood systems begin 

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

 grooves to the pinnula;, but the ambulacra have no sucliers 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 ca-lom. Opposite their ccelomic 

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

 the tubules into the ambulacra! system. The ambulacra! nervous system is 

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

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



