ECHINODERMS. 285 



having the disc distinct from the rays, and the radial 

 canal and nerve inside the calcareous wall of the ravs. 



> 



11. The CRIXOIDEA are fixed by an aboral stalk during a 

 part or a whole of their life. The arms, which may branch, 

 radiate from a central calyx which contains the alimen- 

 tary canal. The mouth is uppermost and at the bottom 

 of a funnel formed by the arms. 



12. The ECHIXOIDEA have a spherical, disc-like, or 

 heart-shaped body, the ambulacra! areas appearing as 

 meridians on its surface. The radial nerves and canals 

 are inside the calcareous plates. 



13. The mouth is surrounded by a complicated jaw 

 apparatus (Aristotle's lantern). 



14. The HOLOTHUROIDEA are elongate and cylindrical, 

 with mouth and vent at the ends; there is no Aristotle's 

 lantern; the madreporite is usually internal. 



