396 DR ARTHUR T. MASTERMAN ON THE 



a morphological standpoint. Of the elements derived from the anterior coelom we can 

 recognise two lateral cceloms right and left. These, from their very first origin, have 

 been exactly equivalent in size, shape, and relationships. Stages A, B and C all 

 indicate clearly the development of these two lateral cceloms, and the only discernible 

 difference between them is the more rapid growth, and hence the larger size, of the left 

 one. It seems difficult to deny to these two a morphological equivalence, and yet the 

 left becomes the water- vascular system, and the right persists as the epigastric ccelom, 

 i.e., the body-cavity of the starfish which lies over the aboral part of the stomach. The 

 conclusion seems to be that the water-vascular system and the epigastric coelom are 

 homologous structures, and have been derived from equivalent portions of the coelom. 

 These two portions, from their position in the larva, seem to be a pair of ccelomic segments 

 lying immediately behind the mouth. 



The central ccelom is lying on the right side, but we have seen that its origin is 

 median in stage B, and the pore-canal, although its rudiment now lies in the middle line, 

 belongs, as we know from other larvae, to the left side. The fourth part of the anterior 

 coelom is that filling the cavity of the pre-oral lobe or the pre-oral ccelom. 



The posterior ccelom, like the anterior, is at first perfectly symmetrical, but the 

 asymmetry is even more striking than in the anterior. Quite early the left posterior 

 element, with its dorsal and ventral horns, becomes much more conspicuous than the small 

 right elementwith only a ventral horn. We have already seen that in the stage D/E these 

 two fuse together to form the hypogastric coelom or main body-cavity of the starfish. 

 How far are these two comparable to the right and left posterior cceloms or ccelomic 

 sacs of Asterina and Asterias pallida ? The only possible reason for doubt arises 

 from their very different fate. The only way in which to reconcile the conclusions here 

 arrived at with the work of Mac Bride and Bury on the fate of the body-cavities is to 

 suppose that the posterior ccelom, with its one dorsal and two ventral diverticula, is merely 

 the left posterior ccdom arising alone at the posterior end of the mesenteron, whilst 

 the right lateral coelom of Cribrella is the right posterior coelom of Asterina, but arising 

 from the anterior coelom and forming the epigastric ccelom. Such a reasoning appears 

 to me a great distortion of the facts in order to fit a special need. Had it been 

 incontrovertibly proved that in other asterids the left and right posterior cceloms become 

 the hypogastric and epigastric cceloms, then it might be advisable to regard Cribrella as 

 one of nature's wilful freaks, somewhat like Kleinenberg's Lopadorhynchus, but the 

 point turns upon what we regard as 'posterior ccelom.' In Cribrella the posterior 

 element cannot possibly be confused with the anterior, because they arise at opposite 

 ends of the mesenteron, and are apart throughout the development. In other asterids, 

 as far as is now known, there is either, as in Asterina, a large anterior ccelom with two 

 arms from which the posterior ccelom is constricted off, or, as in Asterias, there is a large 

 paired ccelom, which in each case has a posterior element cut off from it. In each type 

 the posterior elements join together across the middle line posteriorly, thus obscuring 

 the limits of right and left elements. We therefore have in such types a difficulty at 



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