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THE AMERICAN NATURALIST [Vol. XL VII 



can be nothing else than a heteroradiate echinoderm, and 

 among the heteroradiate echinoderms a holothurian, in 

 which class it comes nearest to certain of the Elpidiidae. 



Affinities of Eldonia. — In Eldonia the body is medusa- 

 .like, circular, bordered with a broad Euphronides-like 

 brim of uniform width; the mouth and anus are near 

 together, the mouth being nearer the center ; the general 

 configuration of the digestive system is very similar to 

 that seen in the endocyclic crinoids ; there are two large 

 many-branched tentacles, one on either side of the mouth. 



Eldonia seems to me to be a pelagic derivative from 

 some elpidiid type; the body has shortened so that the 

 mouth and anus have become closely approximated; the 

 brim surrounding the body has become laterally extended 

 and uniformly developed, so that a swimming bell has 

 resulted. Eldonia is therefore an elpidiid holothurian 

 which has become flattened dorsoventrally and at the 

 same time laterally expanded into a circular form re- 

 sembling that of the medusae. 



We are familiar with just such a transformation in 

 the echinoids; Dendraster, FJclnnarachnius, Arachnoides, 

 etc., are flattened, circular and disk-like, though derived 

 from ovoid, globular or more or less spherical types. In 

 •these the flattening has been in the direction of the radial 

 symmetry so that the oral pole is at or near the center of 

 one surface and the aboral pole at or near the center of 

 the other. In Eldonia the flattening has possibly, though 

 not certainly, been in a plane at right angles to this so 

 that the oral pole is at one edge of the circular disk and 

 the aboral pole at the other. This is only a slight advance 

 over the conditions seen in Benthodytes typicus, so that 

 it need occasion no surprise. 



The reduction of the number of tentacles in Eldonia to 

 two possibly indicates a suppression of three of the 

 radial systems, leaving only two of the original five. 

 Many of the crinoids show a more or less complete reduc- 

 tion of two or three of the radial systems; indeed, in 

 Tetracrimis one is invariably absent. 



