340 



ECHINODERMA. 



walls of the theca are covered with polygonal calcareous plates. 

 Usually the stalk bears five plates, the basalia, and then come five 

 radialia, alternating in order with the basalia (fig. 323). In some 



FIG. 323. Hyocrinus bethleyanus. A, tipper end of stalk with cup, and the bases of 

 the arms; b, basalia; ftr, brachialia; r, radialia. J?, oral surface of cup with 

 mouth, five oralia, and the bases of the arms. 



there is a circle of infrabasalia in a line with the radialia. Fre- 

 quently the elements of the arm, the brachialia, are directly attached 

 to the radials (fig. 323). But often the arm branches once or 

 several times dichotomously, and the first branching takes place at 

 the base, so that the arms seem to spring from the theca. In 

 these cases the first brachialia are considered as part of the theca 

 and are called radialia distichalia (figs. 320, 321). From the arms 

 arise, right and left, a row of pinnulae, lancet-shaped processes 

 supported by calcareous bodies in which the sexual products ripen 

 until freed by dehiscence (fig. 325). 



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

 the theca, is frequently surrounded by five radial plates, the oralia. 

 The mouth, which in contrast to other echinoderms is directed 

 upwards, connects with a spacious digestive tract in which oesopha- 

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

 interradial and near the mouth (fig. 324). Five ambulacral 

 grooves begin at the mouth and extend out on the arms, branching 

 with them and extending to the tips of the pinnulae. These are 



