Factors of Form Regulation ui Harenactis Attenuata 499 



and the number of mesenteries could not be determined with 

 certainty by external examination. 



This form of reduction in the number of tentacles is due, I 

 believe, to the small size of the individual formed from the pieces. 

 In Harenactis, as in other actinians, more or less definite space- 

 relations exist betw^een the mesenteries. This can only mean that 

 the correlations between a mesentery and adjoining regions are 

 of such a nature that other mesenteries are prevented from devel- 

 oping within a certain distance of the already existing mesentery. 

 Consequently if the size of a region in which mesenteries are 

 present undergoes a real physiological decrease, the mesenteries 

 are brought "too near" each other. In cases of this sort, where 

 two organs come into physiological conflict, we commonly find, 

 at least in the lower forms, either that one of them undergoes 

 atrophy, resorption, or separation, or that both are more or less 

 reduced. 



In the two cases under consideration these conditions are pres- 

 ent. The closure of the pieces after section is very slow, conse- 

 quently they remain collapsed for a long time. The body-wall 

 and mesenteries of Harenactis, like those of other actinians with 

 which I have worked, undergo more or less complete atrophy in 

 the absence of the functional stimulus arising from distension of the 

 body by fluid (Cf. Child '04b, '04c, 'o4d, 'o4e, 05a, '08, '09). 

 There is then in such pieces a real physiological decrease in size. 

 I think it probable that under these conditions the mesenteries of 

 the second cycle, i. e., the six pairs of smaller, less highly devel- 

 oped mesenteries, undergo atrophy or degeneration. This of 

 course reduces the number of the inter-mesenterial chambers and 

 consequently the number of tentacles by one-half. By this method 

 of reduction in number every alternate tentacle disappears: appar- 

 ently that is exactly what has occurred in the two pieces with 

 twelve tentacles. One of these pieces produced two more ten- 

 tacles after a considerable time: in this case it may be that the 

 atrophy of some of the second cycle of mesenteries was not com- 

 plete, or else that with the functional growth following renewed 

 distension the reappearance of the mesenteries of the second cycle 

 or of some of them becomes possible again. 



