ACTINOLOGICAL STUDIES. 217 



in accordance with the details sketched above ; seven, or 

 4'24 per cent only showing variation from this typical 

 condition. Many of the specimens were sufficiently large 

 to dissect with knife and scissors, by the aid of a simple 

 dissecting microscope ; the others were cut transversely 

 with a common razor and the details of the arrangement 

 of oesophageal grooves and mesenteries made out from 

 transverse sections. 



In discussing the variation in anatomical characteristics 

 which I have found in Actinia equina, I will briefly refer, 

 for purposes of comparison, to the observations made by 

 other investigators on variations in other species. 



The Hexamerous arrangement of Mesenteries. — A num- 

 ber of species have been described in specimens of which 

 heptamerism, octamerism, or decamerism has obtained, 

 instead of the normal hexamerism, or arrangement of 

 mesenteries in six or multiples of six. 



Last year Prof. McMurrich (1897) described seven 

 specimens of Sagartia spongicola, in which in two the 

 mesenteries were arranged on a heptamerous plan, and 

 in three were arranged on an octamerous plan. In the 

 whole of the 165 specimens of Actinia equina examined 

 by me, there was not one which departed from the 

 hexamerous arrangement. This question is usually 

 decided on the evidence of the "complete" mesenteries 

 only. All of these are of the " incomplete " series at 

 first, and it is merely a question of age as to when they 

 become of the " complete ' series. In several of the 

 specimens of Actinia equina, I found some of the mesen- 

 teries of the same circle complete and some incomplete, 

 sometimes even the same pair has had one mesentery 

 complete and the other incomplete. Again, the attach- 

 ment of the mesenteries to the oesophagus gradually 

 extends downwards from above, and if the arrangement 



