318 Scientific Proceedings, Royal Dublin Society. 



one twenty-two, and one sixteen pairs. The number of directive 

 mesenteries, which always corresponds to the number of oesophageal 

 grooves, is also indefinite ; the specimen with twenty-three pairs of 

 mesenteries had two pairs directive; that with twenty-two pairs 

 of mesenteries had three pairs directive ; while the specimen with 

 thirty pairs of mesenteries had amongst these four pairs directive, 

 and that with sixteen pairs had but one pair directive. The pairs 

 of mesenteries are not all of equal size, but we have been unable 

 to divide them into orders. There is, however, generally a 

 tendency to alternate arrangement. There is nothing constant in 

 the number of mesenteries developed between the directive pairs. 

 In the specimen mentioned as possessing two pairs directive, there 

 were on one side seven, and on the other fourteen pairs, separating 

 the two pairs of directive mesenteries. In the specimen possessing 

 four pairs directive, two of the directive pairs enclosed four 

 ordinary pairs between them, while the other two directive pairs 

 enclosed three ordinary pairs. The two groups of mesenteries so 

 constituted were separated on the one side by ten, and on the 

 other by nine pairs of ordinary mesenteries. All the ordinary 

 mesenteries were in pairs, though some of these were unequal, one 

 of a pair being rudimentary. 



In the specimen possessing three pairs of directive mesenteries 

 (PL iv. figs. 2 and 3) a very remarkable arrangement of mesenteries 

 and grooves was observed. There were in all twenty -two pairs of 

 mesenteries; in a transverse section near the oral disk, at one 

 end of the oesophagus, there was one groove, at the other end of 

 the oesophagus, in close proximity to each other, were two grooves ; 

 corresponding to the groove that stood alone was a pair of 

 directive mesenteries, one of which was rudimentary — a peculiarity 

 observed by Professor Hertwig as occurring in Tealia bunodi- 

 formis ("Challenger" Beport, p. 37). Each of the grooves at 

 the other end of the oesophagus was in connection with a pair 

 of directive mesenteries, and between the latter were three pairs 

 of ordinary mesenteries, two of which pairs reached the oeso- 

 phagus in a section cut at this level. Each of the last mentioned 

 directive pairs were separated from the first mentioned pair of 

 directive mesenteries by eight pairs of ordinary mesenteries. In 

 sections cut lower down across the oesophagus the two neighbour- 

 ing grooves gradually approached one another, and the proximal 



