1898-99. ] THE MESENTERIAL FILAMENTS IN ZOANTHUS SOCIATUS. 401 
was packed with foreign bodies; in buds 3.5mm. in length all the parts 
of the filament occurring in the adult were present. 
It is interesting to note that in the buds of Alcyonaria the same 
acceleration in the development of the ciliated bands has been observed 
by E. B. Wilson (1884), the glandular streaks in the egg embryos of 
these forms developing before the ciliated bands, as in Zoanthids. 
V.—CONCLUSION. 
I have shown above (1) that in adult polyps of Z. soctatus there 
is no histological continuity between the glandular streaks and the cili- 
ated bands; (2) that in egg embryos the glandular streaks develop 
before the ciliated bands make their appearance ; (3) that in the same 
embryos the streaks make their appearance on mesenteries that are not 
connected in any way apparently with the ectoderm; and (4) that in 
bud embryos the ciliated bands appear before the glandular streaks. 
It seems to me from these facts that the ciliated bands must be 
regarded as being ontogenetically distinct from the glandular streaks. 
The two have been very generally regarded as different parts of the 
same structure, but this idea is, I think, untenable. 
If they be recognized as distinct structures, there are no a priori rea- 
sons for regarding both as products of the same germ layer. The 
question of the origin of the filaments, whether from the ectoderm or 
from the endoderm, is one that has frequently been discussed, with 
very varying answers. The majority of authors have regarded both 
parts as ectodermal, or as endodermal, E. B. Wilson having been the 
first, from his studies on the Alcyonaria, to point out the probability 
of the development of the ciliated bands in these forms from the ecto- 
derm and that of the glandular streaks from the endoderm. In my 
studies on the development of the Hexactinians (1891) I reached the 
same conclusion, and the evidence presented above seems to point to 
a similar story in the Zoanthids. 
However, there is a more fundamental consideration at the base of 
all questions as to ectodermal and endodermal origin in the Ccelentera. 
Is there a sufficient fixity of the germ layers in these forms, whether the 
layers are regarded from the morphological or the physiological stand- 
point, to warrant the importance which has generally been attached to 
them? The germ layers have evolved; like other structures they have 
