Chap. IX. ] 



CALYX OF CRINOIDS. 



293 



Echinoderms ; the plate is, in the Crinoids, modified 

 by the large share in its formation that is taken by the 

 topmost plate of the stalk. 

 In the development of An- 

 tedon, the interradially- 

 placed tmsals become ob- 

 scured, but in other forms 

 among the Crinoids they are 

 evident throughout life. 

 Careful investigation into the 

 structure of the skeleton of 

 Asteroids and Ophiuroids 



Fig. 122. Pentacrinus Wyville-Thomsoni. (Natural size.) 



reveals the presence of the plates to which our atten- 

 tion has just been directed. 



The following are the more important points in 

 the structure of the skeleton of the Starfish; the 

 arms, or rays, are made up of a number of ossicles 



