246 ECHINODERMA. 



The Holothurians have no ab-oral system of plates, and the radial 

 symmetry does not affect the reproductive organs. These two negative 

 characters, combined with some positive ones, may indicate that the 

 Holothurians are primitive, and, as is certainly suggested by their 

 external appearance, have affinities with the supposed "worm-like" 

 ancestors of Echinoderms. 



Again, some members of the heterogeneous class of Cystoids are 

 extremely primitive, but differ from the Holothurians in the possession 

 of an ab-oral system of plates, alternately radial and inter-radial. From 

 this primitive Cystoidean stock, two branches diverge. The one leads 

 to the sessile Cystoids, Blastoids, and Crinoids (Pelmatozoa) ; the other 

 to the free Echinoidea, Asteroidea, and Ophiuroidea. Of these, the 

 existing Asteroidea and Ophiuroidea are late divergences from a. 

 common stock. 



