THE EDWARDSIA-STAaE OF LEBRUJSTA. 303 



absolutely I can come to no other coQcIasioD, and, of course, the 

 correctness or otherwise of the explanation of many of the other 

 peculiar features is determined thereby. The view has the 

 merit of placing the conditions in strict harmony with what 

 occurs in many Enterocoela. 



The limiting layer of the larval ecelomic spaces in continuity 

 "with the lining of the archenteron must be considered as the 

 equivalent of the wall of the ccelomic pouches or coelomie split- 

 tings of the higher Metazoa. Here it is usually regarded as 

 mesoderm or mesothelium, the latter term being employed ia 

 order to distinguish it from the mesoderm — " mesenchyme " — 

 arising by immigration. 



The tendenc}^ however, is to regard still as endoderm the 

 walls of all tlie outgrowths from the archenteron and their de- 

 rivatives. Thus in the latest important work on the morphology 

 of the Echinoderms, Dr. H. L. Clark (1898) describes as meso- 

 derm only the tissue of mesenchymal origin, and this plays a 

 very insignificant part in the adult structures. Under endoderm 

 he includes all the derivatives of the archenteron and its hydroen- 

 terocoel, embracing amongst other structures the lining of the 

 digestive tract with most of the oesophagus ; all the muscles of 

 the body-wall and gut ; the peritoneal lining of the body-cavity 

 and epithelial covering for the various organs contained in it, as 

 well as the genital organs and their ducts. 



Homologizing the limiting layer of the coelomie pouches 

 in Lehrunia with the peritoneal epitheliatn of the body-cavity 

 of the higher Metazoa, we may, with Clark, still regard it as 

 endoderm, or, following the more usually accepted terminology, 

 speak of it as mesoderm ; and, as in all the Enterocoela, it 

 gives origin to the principal muscular system and the gonads. 

 The portion adjacent to the column- wall and that covering 

 the two faces of the mesenteries would correspond with the 

 somatic, and the remainder with the splanchnic layer of the 

 higher Metazoa, and it is the former which remains as the 

 epithelial lining of the major part of the adult mesenteric 

 chambers. Below the oesophageal region it is the splanchnic 

 layer which disappears during disintegration, but above, this 

 constitutes the ccelomic lining of the gullet. 



What, then, of the vacuolated mass of tissue which probably 

 entirely disappears in the adult ? It is the tissue within which 



