No. 560] CAMBRIAN HOLOTHUBIANS 497 



In certain forms among the Comasteridae, as, for in- 

 stance, in Comatula micraster, ambulacral grooves, nerves 

 and tentacles may be entirely absent from six out of the 

 ten arms, or from three out of the five rays, leaving, as in 

 Eldonia, only two of the original five divisions function- 

 ing normally, and these two may be three times as large 

 as the others. 



If we can assume that the two tentacles of Eldonia in- 

 dicate a suppression of three of the original five radial 

 systems, or a carrying out to completion of the condition 

 already far advanced in many of the Elpidiidae, a reason- 

 able explanation of the structure of Eldonia becomes a 

 relatively simple matter. 



If we take a form like Scytoplanes typicus or Euphro- 

 nides tanneri and shorten the body so as to bring the 

 mouth and anus near together, giving the digestive tube 

 exactly the same shape that it assumes in the so-called 

 endocyclic crinoids, the two radial muscles would form 

 a circular band of concentric muscle fibers just beyond 

 the enteric canal, exactly as we see them in Eldonia. 



Ordinarily among the echinoderms the mouth is at one 

 pole of the radial symmetry and the anus is at the other. 

 In the recent crinoids the anus has become entirely dis- 

 sociated from the aboral radial pole and has migrated to 

 a position near the mouth. It is thus evident that the 

 connection between the anus and the aboral pole is not 

 absolutely unchangeable. 



The water vascular system centers, in all echinoderms, 

 in a ring about the oesophagus, from which (usually) 

 five radial canals are given off. How then can we ac- 

 count for the small central ring in the center of Eldonia, 

 far removed from the mouth? The first question to be 

 answered is whether the arrangement of the water vas- 

 cular system about the mouth is really fundamental, or 

 whether it is merely a matter of mechanical convenience 

 when the mouth happens to be, as it usually is, at or near 

 one of the apices of the pentamerous symmetry. 



Now among the crinoids there are two families, the 



