No. 560] CAMBRIAN HOLOTHUBIANS 



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very small, also concentric, apical water vascular ring 

 with exceedingly long tubes reaching out into the margi- 

 nal tube feet, fused together into a broad and uniform 

 marginal brim. 



In Eldonia the radial canals so-called are the canals of 

 the podia transformed into a system of braces, exactly as 

 they are transformed into a system of braces in Euphro- 

 nides tanneri, Benthodytcs typica, and many similar 

 forms ; in these species, which live supported upon ooze 

 and have developed a broad brim about their body so 

 that they will not sink into it, the ring canal of the water 

 vascular system retains its original position about the 

 oesophagus, while in Eldonia, which floats free in the 

 water, and possesses a medusoid body form, the canals 

 have become enormously elongated and of uniform length 

 all around, serving as body supports (like the ribs of an 

 umbrella) instead of merely as supports for an expanded 

 brim; and, as a necessary result of the change in the 

 mechanics of the body, the central ring of the water 

 vascular system has migrated from its original position 

 about the gullet to an apical position in the center of the 

 apical portion of the animal, equidistant from the border 

 on every side. 



The entire dissociation of the water vascular system 

 from the mouth is the most difficult thing to explain in 

 Eldonia. But, after all, this is not without a parallel. In 

 the bilaterally symmetrical invertebrates one of the most 

 fundamental structures is the nerve ring about the oesoph- 

 agus, consisting of the supraoesophageal ganglion, the 

 two circumcesophageal ganglionic connectives and the 

 suboesophageal ganglion. The relationship of these 

 nerves is entirely changed in the crinoids ; here we find a 



