No. 6.] THE MORPHOLOGY OF THE ACTIXOZOA. 



J67 



ever, have assumed an appearance much more like those of the 

 adult, and the histological differentiation of their epithelium is 

 quite pronounced. The mesenteries figured are the same as 

 those shown in Fig. 10, but of the opposite side of the body. 

 It will be seen that in mesentery VI all traces of the glandular 

 streak have vanished, but in mesentery V the streak is still 

 persistent, and indeed has undergone a progressive develop- 

 ment, just as those of the perfect mesenteries. That this is 

 not because the larva is the young of a macrocnemic species is 



Fig. II. 



-Transverse section through a portion of the column of a zoanthid embryo 

 somewhat older than that from which Fig. lo is taken. 



shown by the fact that it is not mesentery VI, the additional 

 perfect mesentery, in these species, which has retained its fila- 

 ment, but mesentery V. Probably later stages would show a 

 disappearance of the filament of this mesentery also, but the 

 point which is of concern is the fact of the development of fila- 

 ments on these imperfect mesenteries whose epithelium is, so 

 far as can be ascertained, at no point in contact with ectoderm.^ 



^ Attention may be called to the fact that the discovery of filaments in these 

 mesenteries serves to emphasize the correctness of the conclusion as to the order 

 of the appearance of the mesenteries in the Zoantheae which has been stated by 

 Boveri ('89) and myself ('91a), and I may add that indications of filaments in the 

 microdirectives can also be distinguished, though they are much less evident than 

 those of V and VI, possibly on account of an earlier degeneration. 



