THE EUWAEDSIA-STAaE OF LEBRUNIA. 291 



oesophagus. The greater part of the limiting layer of the spaces 

 persists as the unilaminar epithelium of the gastro-coelonaie 

 cavity of the adult. 



4. Mesenteries. 



Eight mesenteries are present in each stage examined, all 

 perfect in the upper oesophageal region ; but only two, the sulco- 

 lateral pair, remain united as far as the lower edge of the ceso- 

 phagus. All may extend to tlie aboral termination of the 

 internal cavity, but the sulcular directives sometimes cease a 

 little in advance of the others. Tl]e four pairs subdivide the 

 upper oesophageal region into eight chambers, four larger alter- 

 nating with four slightly smaller, arranged so as to present a 

 perfectly tetrameral, radial symmetry (PL 19. fig. 16), in corre- 

 spondence witli that of the tentacles on their first appearance. As 

 the three pairs become free the transverse sections assume a 

 distinctly bilateral symmetry. In their free condition the sulco- 

 laterals continue for a short distance still larger than the rest, 

 but below all the eight are practically equal, and very short in 

 their centripetal extent. 



It is remarkable that no increase in the number of mesenteries 

 takes place between the earliest and the latest larvge, represent- 

 ing a period of about five days. This is in harmony with the 

 numerical condition of the tentacles, but the development of 

 the mesenteries precedes that of the tentacles in all known 

 cases. 



In !-ome examples the two- sulcar directives, in the proximal 

 region, become united along their free edges. 



The mesogloea of the mesenteries is usually broad at its origin 

 in the column-wall, but is extremely thin beyond. In the 

 earliest stages traces of muscular elements are indicated only in 

 the basilar region, but later, transverse sections of very delicate 

 longitudinal fibrils are seen along one face, just sufficiently well- 

 developed to enable the paired arrangement of the mesenteries 

 to be established (fig. 16). This follows the usual order, which 

 is the same as that in Edwardsia, namely, the retractor muscles 

 on the directives are on the faces of the mesenteries turned away 

 from one another, while in the two lateral pairs they are on the 

 faces turned towards one of the pair of directives which, by thi? 

 means, if by no other, can be distinguished as the sulcar 

 directives. 



