278 B. W. MACBRIDB. 



which still persist in the adult as sharply separated cavities, 

 only the most posterior pair, viz. the right and left posterior 

 coeloms (oral and aboral) of the adult having partially fused 

 with each other. The development of Antedon seems to answer 

 this question in the affirmative. In its case the hydrocoele is 

 budded oflF quite independently of the posterior coelomic sacs. 



Adopting, then, the view that the coelomic sacs of the Ente- 

 ropneusta and Asterids correspond, we find that the hydroccele 

 represents the collar cavity. Now in Cephalodiscus the collar 

 cavities are produced into long pinnately branched tentacles, 

 comparable to the radial water-vascular canals, and further a 

 branch from the central nervous system accompanies each 

 tentacle, just as the radial nerves accompany the radial canals 

 in Echinoderms. Now, if we suppose that the two hydrocoeles 

 of Asterina were equally developed and approximated in the 

 mid-dorsal line, the fusion of the anterior portion of the two 

 nerve "rings," which of course would in this case be only open 

 curves (since a ring-form is attained through the preponderating 

 growth of one side) would give rise to a mid-dorsal nervous 

 system like that of Cephalodiscus. Nor is that all ; Professor 

 Spengel (21) has shown in his monograph of the Enteropneusta 

 that the currents in the proboscis-pore and collar-pore are 

 inwards, and that by this means the animal inflates the proboscis 

 and collar so as to render them efficient locomotor organs. 

 We have seen that the function of the stone-canal is a similar 

 one. 



We conclude, then, that the free-swimming ancestor of 

 Echinoderms, for which we may adopt the name Dipleurula, 

 and the Tornaria ancestor of Balanoglossus, were closely allied. 

 This involves the assumption that they were allied to the Pro- 

 tochordata, for, as I have elsewhere stated (16), Professor 

 SpengeFs attempt to refute the Chordate affinities of Balano- 

 glossus has been, in my opinion, futile. Although it may seem 

 somewhat fanciful, I cannot help seeing hints of Vertebrate 

 peculiarities in the anatomy of Echinoderms. Where else 

 among all animals of higher grade than the Coelenterates do 

 we find calcareous ossicles in the dermis ? Where else 



