l$6 McMURRICH. [VOL. II. 



These observations show reason for believing that in the 

 Kdwardsiae there is an intimate relation between the number 

 of tentacles and that of the mesenteries, and that when there 

 are more than eight tentacles there is a strong probability that 

 a number of short mesenteries are also present in the upper 

 part of the column. It is a general rule in the Actininae that 

 the number of tentacles in the fully developed condition is 

 double that of the pairs of mesenteries or, in other words, that 

 there is a tentacle corresponding to each endocoel and each 

 exocoel, the number of tentacles being equal to the total num- 

 ber of mesenteries. Exceptions, due to a lack of development 

 of the full complement of tentacles, are of common occurrence, 

 in many cases probably owing to the specimens examined not 

 having reached their full development, though even in some 

 adults, apparently, the number of tentacles never reaches that 

 of the mesenteries, as is the case, for instance, in Pcachia 

 Jiastata, which, with twenty mesenteries, never has more than 

 twelve tentacles. 



The rule may be better expressed by saying that tlic number 

 of the tentacles never exceeds that of the mesenteries, and when 

 an apparent exception to this occurs the presumption is that 

 closer examination will reveal the existence of small mesen- 

 teries limited to the upper part of the column and in sufficient 

 numbers to fulfill the requirements of the rule. 



Acting on this supposition, I have made a further study of 

 the upper portion of the column of Halcurias filatns, a form 

 which I have already described as possessing twenty mesen- 

 teries and a number of tentacles considerably in excess of that 

 of the mesenteries, having been estimated in one specimen 

 ('93) to be about seventy, and in another ('98) to be about 

 sixty. Sections showed, as I had expected them to do, the 

 presence of a number of short and narrow mesenteries in the 

 upper part of the column, the number of these pins the twenty 

 perfect mesenteries being equal to the total number of the 

 tentacles, which proved to be sixty-eight. 



The sections also revealed, however, a peculiarity which I 

 had not expected, and which, as may be seen from Fig. i, con- 

 sisted in the short, narrow mesenteries being developed in the 



