322 McMURRICH. [Vol. IV. 



and are perhaps to be referred to the early appearance of the 

 mesenterial filaments. 



It must be remembered, however, that there are different 

 opinions as to the nature of the mesenterial filaments in the 

 Actinozoa, some authors considering them to be endodermal and 

 others ectodermal. As stated above, they appear to be ectoder- 

 mal in Rhodactis, at least the median portion, which is all that 

 is developed, does. At the same time I must refrain from ex- 

 pressing any certainty on this point, (i) on account of the slight 

 difference in histology between the ectoderm of the stomato- 

 daeum and of the filaments, and (2) because it seemed, in the 

 case of a very young filament of the second pair of mesenteries, 

 that, in a series of transverse sections, there was a very short 

 streak of indifferent cells intervening between the stomatodaeum 

 and the filament, interrupting the continuity of these struc- 

 tures. If this be the case, it would seem that the filaments, or 

 at least their median portion, were endodermal, which seems 

 to be the case with those of Aidactinia, to be described later. 



3. Later Stages in Aulactinia. 



In the youngest embryo of Aidactinia which I possess, the 

 endoderm presents certain peculiarities which allow us to infer 

 the mode of development in the earlier stages to a certain ex- 

 tent. There are already eight perfect mesenteries, only two of 

 which — those of the first pair — are provided with mesenterial 

 filaments. The endoderm has arranged itself into a somewhat 

 definite layer, but lying scattered about in the body cavity of 

 the embryo are numerous somewhat large cellular elements and 

 yolk-granules. The endoderm is in a stage of differentiation 

 which corresponds fairly well with that represented in Rejnlla 

 by E. B. Wilson ('83) in his Fig. 125, PL 57, From the occur- 

 rence of these cellular elements, it seems probable that the 

 formation of the germ-layers took place in a manner similar 

 to what is found in the Alcyonarians, rather than like what I 

 have described for Metridiiim. 



The stomatodaeum in these youngest larvae is elongated 

 throughout its entire length in a direction at right angles to the 

 elongation noticed in its lower part in Rhodactis, so that a line 

 drawn in the axis of the elongation passes through the inter- 



