258 JELLY-FISH, STAR-FISH, AND SEA-UKCHINS. 



nccted together so as to compose a series of struc- 

 tures not unlike the couples of an ordinary roof. 

 These so-called ambulacral plates rest on liorizontal 

 spine bearing plates, from which other larger plates 

 extend u[)\vards to form the sides of the arms. 



In a living Star-fish the tube-feet or pedicels 

 already mentioned are seen projecting from each 

 side of the ambulacral groove ; and, Avith the excep- 

 tion of a few at the tip of each arm, all the tube- 



Fig. 35. — The terminal portion of a tube foot (in.Tgnified). 



feet terminate in a well-formed sucker, by means of 

 which they can be firmly fixed to a flat surface 

 (Fig. 35). 



If we wish to understand the structure and 

 mechanism of this locomotor or ambulacral system 

 — which, I may observe in passing, is of special 

 interest from the fact that as a mechanism it is 

 unique in the animal kingdom — w^e must resort to 

 dissection. We then find that each of the tube-feet 



