PHYSIOLOGY OF THE INVERTEBRATA. 



383 



{d) The OpMuridea. — In the brittle-stars the ambulacral 

 feet are only rudimentary, although exceedingly active; they 

 are devoid of suckers ; and their mode of protrusion and re- 

 traction is exactly like that of Astropecten, but more rapid in 



>< 



I 



« 

 .< 



w 



Z 

 < 



a 



o 



S 

 •^ 



o 





action. These animals are much the most actively locomotive 

 of all the starfishes ; " and the reason is, that having discarded 

 the method of crawling by. the ambulacral system, which is 

 common to nearly all the other Echinoderms, they have 

 adopted instead a completely new, and a much more effectual 



