No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 221 
The view that the gastric lining of the cesophagus and peris- 
tome is ectoderm can only be held by supposing the epithelium 
in question to have suffered a great histological change. (It is 
possible that a very careful histological examination of the epi- 
thelium, x, would show that it does not differ so much from the 
ectoderm as appears to be the case.) But though this tract of 
epithelium cannot be said to resemble the ectoderm, it differs 
quite as much from the undoubted endoderm. The endoderm 
of the adult, Figs. 51 and 52, is made up of large irregularly 
columnar cells, packed with ‘yellow cells,” the vacuoles present 
in the larval endoderm being confined to special localities. The 
only apparent alternative to the view offered is that while the 
first twelve mesenteries are provided with ectodermal filaments, 
the filaments of all subsequent mesenteries are endoderm. 
Since the several orders of filaments in the adult differ only 
as regards size, and since even this difference is a transient 
one, owing to the constant transformation of incomplete into 
complete mesenteries during the growth of the coral, it seems 
improbable that such precisely similar organs should be formed 
by both layers. 
On the other hand, E. B. Wilson (8) came to the conclusion 
that the dorsal pair of filaments in the Alcyonaria were ectoder- 
mal lobes, but that the remaining six filaments were purely 
endodermal. The two kinds of filaments in these polyps have, 
however, a very different histological structure, with which is 
associated a division of labor. 
Von Heider (11) several years ago decided that the filaments 
of Cerzanthus were ectodermal. He reached his decision by a 
histological study of the adult, and though this method is incon- 
clusive, I am not surprised after studying myself some immature 
specimens of Cerianthus, that he came to this view. The 
Hertwigs-in their classical work on the actiniz pointed out that 
embryological deductions based on adult histology are not very 
reliable, and also brought forward as an objection to von Heider’s 
view, the existence of filaments in the actiniz generally on 
incomplete mesenteries, 
