128 McMURRICH. [Vol V. 



probable, though they had no definite evidence on the subject, 

 that the fourth and second pairs of Lacaze-Duthiers' description 

 should be reversed. According to this succession, the second 

 pair of mesenteries appears in the larger chamber formed by 

 the first mesenteries, dividing it into a dorsal and two dorso- 

 lateral chambers ; the third mesenteries develop in the ventral 

 chamber and become the ventral directives ; while the fourth 

 mesenteries appear in the dorsal chamber, and become the 

 dorsal directives. The first reason the Hertwigs give in sup- 

 port of their supposition is, that since all the later mesenteries 

 are known to develop in pairs, it is probable that the first 

 eight mesenteries likewise did so. We know now that the four 

 mesenteries which convert the Edwardsia stage into the Hex- 

 actinian are in certain cases not developed in pairs, and there- 

 fore the Hertwigs' first argument must be considered as having 

 little weight. Their second reason seems, however, stronger. 

 They point out that the filaments of the mesenteries which La- 

 caze-Duthiers considers to be the fourth pair develop earlier 

 than those of either his second or his third pair. It seems to 

 be a general rule that the succession of development of the 

 filaments corresponds with that of the mesenteries, and there- 

 fore it would seem probable that there is an error in Lacaze- 

 Duthiers' observations, explainable by the fact that he made no 

 use of sections. 



This probability is rendered stronger by the observations of 

 H. V. Wilson ('88) on Mankind, mine ('90) on Rhodactis, and 

 those of Faurot ('90) on Halcampa, in all of which cases it 

 seems certain that the fourth pair of mesenteries are the dorsal 

 directives. 



Haddon ('89), however, states very positively that F. Dixon, 

 working under his direction, has been able to confirm by the 

 study of sections Lacaze-Duthiers' observations. The only 

 conclusion that seems possible is, that there may be consider- 

 able variation in the order of appearance of the first eight mes- 

 enteries. I believe from the occurrence of the order which I 

 have found in Rhodactis in such widely separated forms as that 

 genus, the Madrepores and the Halcampidae, that it must be 

 regarded as the more primitive and normal. Lacaze-Duthiers' 

 arrangement may be explained, not by a supposed transforma- 

 tion of the second pair of mesenteries into the dorsal direc- 



