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THE AMEBIC AN NATURALIST [Vol. XLVII 



Comasteridae and the Uintacrinidae, in which the mouth, 

 instead of being as usual in the center of the ventral sur- 

 face of the disk, is lateral, situated typically on the very 

 edge of the ventral surface of the disk, between the bases 

 of two of the arm groups. In these families the tubes 

 of the water vascular system, above which ambulacral 

 grooves usually, though not always, run, instead of con- 

 verging in five large vessels to the circumoral ring lead 

 from the arms to a large trunk vessel which runs around 

 the periphery of the disk with the anal tube instead of 

 the mouth as its center. This large peripheral ring, is 

 interrupted posteriorly, and the mouth passes through it 

 anteriorly; but it indicates a tendency for the water vas- 

 cular system to transform from a ring about the mouth 

 to a ring about the anal tube, or more correctly, into a 

 ring about the ventral pole of the body regardless of the 

 position of the mouth. In this connection it would be 

 interesting to determine if in the Comasteridae the so- 

 called stone canals were confined to the circumoral ring, 

 or if they showed a tendency to migrate secondarily 

 along the peripheral water tube. 



With the assumption by Eldonia of the circular form 

 and the spiral digestive tube the muscles assumed a con- 

 centric arrangement. Probably at the same time the 

 water tubes, following the course of the muscles, also at- 

 tained the form of a peripheral canal, after the same 

 manner as we see almost consummated in the Comas- 

 terida?. The peripheral water tubes in Eldonia serve 

 largely as braces to bind the animal together, just as they 

 serve as braces in the marginal brim of Euphronides 

 tanneri. This function would, for mechanical reasons, 

 induce a diminution in the diameter of the central ring, 

 in order that they might function to the best advantage ; 

 but the muscular ring, in order to preserve a maximum 

 availability for expansion and contraction, would remain 

 with the greatest possible diameter. Thus we should 

 theoretically reach a condition precisely like that seen in 

 Eldonia; a very large concentric muscular ring, and a 



