PHYSIOLOGY Ob THE IMVERTEBRATA. 



383 



{(}) The Ophiuridca. — In the brittle-stars the anibulacral 

 leet arc only rudimentary, although exceedingly active; they 

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

 traction is exactly like that of Astropccicn^ but more rapid in 



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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 



