420 CASWELL GRAVE 



terocoeles so universally found in echinoderm larva, but, since 

 the subsequent history of this pouch in Ophiura showed that it 

 passes wholly and exclusively into the adult hypogastric coelom 

 and takes no part in the formation of the epigastric coelom, it 

 became apparent, either that the suggested homology of this 

 larval pouch with the posterior pair of pouches was wrong, or 

 that the material representing the right member of the pair of 

 enterocoeles has, in consequence of its fusion with the left, been 

 completely changed in both position and function, for the usual 

 history of the pair of posterior enterocoeles of echinoderm 

 larvae is that the left member becomes the hypogastric coelom 

 of the adult and the right member differentiates into the epigastric 

 coelom. In larvae of a slightly later stage (stage C) the epigastric 

 coelom was found in its proper position over the right side of 

 the stomach portion of the alimentary tract and its form and 

 structure were such as to indicate that it had not been derived 

 from any part of the large ventral anlagen of the hypogastric 

 pouch. The only suggestion as to the origin of the epigastric 

 coelom, that seemed possible from the structure of the larvae 

 of these two stages, was that the right anterior enterocoele, 

 plainly present in stage B and not evident in stage C, had 

 migrated to a new position on the right side of the stomach, 

 which, in stage B, was unoccupied by any coelomic sti-ucture 

 but which in stage C contained a small epigastric body cavity. 

 The large five lobed structure which in stage B was originat- 

 ing from the left anterior part of the large ventral hypogastric 

 pouch was evidently to be homologized with the hydrocoele. 



Passing now to a consideration of the results of the study 

 of material secured since the publication of the first paper, it 

 is perfectly clear that the epigastric coelom of Ophiura is not 

 derived from a displaced and metamorphosed right anterior 

 enterocoele, but is differentiated, in an even more unexpected 

 and unprecedented way, by an invagination of that portion of 

 the body wall (ectoderm) of the larva which immediately over- 

 lies the right side of the stomach portion of the alimentary 

 tract. The drawings of both the reconstruction and the sec- 

 tion of larvae which show this structure (figs. 4 and 5) were 



