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XVII. 



ON THE AEEANGEMENT OF THE MESENTEEIES IN THE 

 GENUS SAGARTIA, GOSSE. By FEANCIS DIXON. 

 (Plates I. and II.) 



[Communicated by Professor A. C. Haddon.] 

 [Eead April 18, 1888.] 



The following investigations were undertaken to try to explain a 

 fact which, had been observed by my brother (antea, p. Ill), that 

 some species of Grosse's genus Sagartia possessed sometimes but a 

 single oesophageal groove. 



When considered in connexion with the current opinion that a 

 hexamerous arrangement of mesenteries prevails in all the more 

 common Actiniae, the results arrived at were more surprising even 

 than might have been expected from the variability in the number 

 of oesophageal grooves. For it was found that not only the 

 number of pairs of directive mesenteries varied as the number of 

 grooves, but that the number of perfect mesenteries reaching the 

 oesophagus varied in each case, apparently with the size of the 

 specimen examined, a completely hexamerous arrangement being, in 

 most cases, not recognizable. The only regularity common to all 

 the specimens examined was that just mentioned, viz. that the 

 number of pairs of directive mesenteries corresponded to the num- 

 ber of oesophageal grooves. In most cases the number of mesen- 

 teries reaching the oesophagus near the oral disk was considerably 

 greater than lower down near the base of the oesophagus. In all 

 the larger specimens, at all events, the oesophageal ridges corre- 

 sponded to the lines of insertion of the mesenteries ; and, as my 

 brother has pointed out that the oesophageal ridges correspond to the 

 radii on the disk, it would seem that the radii on the disk mark the 

 position of the mesenteries. Whether the arrangement of the 

 mesenteries is at first regular, and if so, how it afterwards becomes 

 irregular, it is impossible to say at present, owing to an insufficient 

 number of specimens of different ages having been examined. 



