THYONE. 77 



Nervous System. This cannot be satisfactorily studied in 

 dissections. There are five radial nerves and a circular ring. 

 The cords are embedded in the body-wall and are hard to find. 



The classes of the Echinodermata show exceptionally well 

 how a general type of structure may be retained and still modi- 

 fied in certain regards for special habits. Compare, for instance, 

 the feeding habits of the starfish, sea-urchin, and sea-cucumber. 



