No. 3.] MORPHOLOGY OF THE ACTINOZOA. 319 



embedded in the endoderm, which, in the upper part of the body, 

 does not yet show any signs of forming a definite layer, although 

 below this, differentiation is accomplished, and a well-marked 

 cavity e-\ists. There are consequently in the upper part of the 

 body as yet no intermesenterial cavities. In its upper part the 

 stomatodasum is circular in section, but below it is elongated 

 in the plane of the two mesenteries, which are situated nearly 

 opposite each other. 



The mesenteries of the second pair make their appearance 

 simultaneously and without any migration and apposition of the 

 stomatodaeum to their line of origin. They are formed in the 

 larger space, cut off by the two previously formed mesenteries, 

 and gradually increase in size until they reach the stomatodaeum, 

 with which they unite. 



The third and fourth pairs of mesenteries also appear simul- 

 taneously (Fig. 16), the third pair being in the interval between 

 the mesenteries of the first pair ; i.e. at the ventral surface of 

 the embryo, and the fourth pair in the interval between the 

 mesenteries of the second pair, or at the dorsal surface of 

 the embryo. In the oldest embryo of Rhodactis which I possess 

 these mesenteries are not yet perfect, and the longitudinal mus- 

 cles have not developed, so that it is impossible to get definite 

 evidence as to which pairs of mesenteries are really the direc- 

 tives. The manner of flattening of the stomatodaeum in its lower 

 part would suggest that each pair of directives consists of one 

 mesentery of the first pair and one of the second ; but it is to be 

 noticed that the upper part of the stomatodaeum does not take 

 part in this flattening, which is probably to be considered as a 

 temporary condition. It seems more probable that the future 

 and permanent flattening of the stomatodaeum is at right angles 

 to the one existing in these early embryos of Rhodactis, and 

 that the third and fourth pair of mesenteries will form the direc- 

 tives of the adult. 



H. V, Wilson has described in Manicina a peculiar reflection 

 of the ectoderm of the stomatodaeum upon the external {i.e. 

 normally endodermal) surface of that organ, and considers it to 

 be concerned with the formation of the mesenterial filaments, 

 with the exception of those of the first pair of mesenteries. In 

 my specimens of Rhodactis the formation of the mesenterial 

 filaments differs considerably from what has been described for 



