Inter -relationa of the Classes of the Echinodermata. 211 



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The typical arrangement in an Echinoid is that the ambu- 

 lacra extend from the mouth to the boundaries of the caly- 

 cinal area ; in Asteroids and Ophiuroids the great develop- 

 ment of additional plates causes the ambulacra to be confined 

 to the oral aspect of the body. I am unable to find a very 

 satisfactory term for this arrangement, but I propose, pro- 

 visionally at any rate, to speak of it as zygopodous in the 

 Urchin and azygopodous in the Starfish. Whatever we do, 

 we must he careful not to use the term brachiate ; for the 

 arms (brachia) of a Crinoid are formed by addition to the 



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