220 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



arranged opposite each other, is shown in PI. XX., figs. 

 4 and 5. The one differs from the other in the relative 

 position of the oesophageal grooves (a>. g.). In one (fig. 4) 

 they are close together and are associated with adjacent 

 pairs of mesenteries of the first order ; and in the other 

 (fig. 5) they are separated from each other by three pairs 

 of complete mesenteries of the first order on the one side, 

 and by one pair on the other. In both cases the first 

 cycle of mesenteries only is complete. 



Directive Mesenteries. — In every instance of the 165, I 

 found each oesophageal groove in relation with a pair of 

 directive mesenteries, and I never found directive mesen- 

 teries otherwise than in relation with oesophageal grooves. 

 Of the 131 specimens of Metridium marginatum examined 

 by Parker (1897), he records that in every case, whether 

 possessing one, two or three oesophageal grooves, there 

 was always associated with each a pair of directive mesen- 

 teries. This exact correlation of the two structures is, 

 however, not the case in all Actinians. McMurrich 

 (1891) and others have shown that in certain forms 

 (Peachia, Oractis, Ptychodactis) directive mesenteries 

 occur, and no oesophageal grooves are discernable. But 

 this want of correlation between the two sets of structures 

 is very exceptional, and the great majority of the species 

 investigated show the same exact correlation as is seen 

 in Metridium marginatum and Actinia equina. 



My observations on the whole tend to show that 

 Actinia equina has a constancy of character not possessed 

 by many other Actinians, and it is of importance to find 

 a dominant species, as A. equina undoubtedly is, so free 

 from variation as this record shows. That certain species 

 are not so liable to variation as others, and are much more 

 constant in their essential characters, has been shown by 

 other investigators to be the case. Even species of the 



