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THE AMEBIC AN NATURALIST [Vol. XLVII 



strands parallel to, instead of passing horizontally 

 around, the gullet. Surely if such a fundamental re- 

 arrangement can take place in the nervous system we can 

 not be surprised in seeing the water vascular system 

 become entirely dissociated from the mouth. 



Pelagic animals tend to become delicate and translu- 

 cent, and if belonging to groups with a more or less cal- 

 cified (or chitinous) skeleton, to reduce that skeleton to 

 a minimum or to dispense with it entirely. Thus the 

 entire absence of any trace of a skeleton in Eldonia does 

 not prevent us from suggesting an affinity with the holo- 

 thurians, more especially as the recent pelagic holothu- 

 rian Pelagothuria has no trace whatever of calcareous 

 elements. 



Dr. Clark remarks that the oral tentacles of Eldonia 

 are suggestive of the marginal clusters of Lucemaria 

 and its allies, or perhaps are not fundamentally different 

 from those of some rhizostomous medusae. This would 

 naturally be the case no matter to what group Eldonia 

 belonged. The tentacles of Eldonia undoubtedly are sub- 

 ject to the same mechanical and physical forces as are 

 those of the pelagic medusae, and this would be amply 

 sufficient to induce a strongly marked parallelism, no 

 matter what their ultimate origin might have been. 



The body of Pelagothuria is tubular, with the mouth 

 at one end and the anal opening at the other; its ali- 

 mentary canal is in loops (a long-drawn-out spiral) ; the 

 swimming organ is merely an expansion of the oral disk. 

 Pelagothuria, therefore, though similarly pelagic, is radi- 

 cally different from Eldonia, derived from a radically 

 different stock. 



The embryology and metamorphosis of the echino- 

 derms lead us to believe that they were derived from 

 phyllopod crustacean ancestry through the barnacles as 

 a result of the sudden suppression of one half of the body 

 and the consequent assumption of a circular body form. 

 Since there are only two tentacles in Eldonia, we might 

 suggest the remote possibility that Eldonia may have 



