DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
771 
and the presence of a rigid axis would in such a case be disadvantageous. Hence we 
can see how, by natural selection, the posterior muscular part of the peduncle might 
be constantly increased in size and importance, accompanied by a corresponding 
reduction of the axis. If this process were continued until the axis disappeared, a 
condition would result like that shown in Renilla amethystina, and by a further 
reduction the structure of R. reniformis would be attained, in which the dorsal 
and ventral chambers and the septum transversale alone remained. 
The foregoing considerations strongly suggest that the peculiarities in the structure 
and formation of the peduncular septum may be in some way a result of the former 
existence of an axis. It is, however, useless to speculate on this matter so long as 
the development of the axis in the typical Pennatulids is unknown ; and in regard to 
this, in Kolliker’s words, “mangeln alle und jede Erfalirungen.” 
Two entirely different views of the Pennatulid axis have been entertained. 
Kolliker, on the one hand, regards it as of mesodermic origin, the mesodermic 
elements being supposed to be originally derived from the entoderm. To quote his 
own words (‘ Pennatuliden,’ p. 428) : “Anders bei der Ivalkaxe, denn hier spielt ein 
osteoblastenahnliche Zellenlage, deren Abstammung von dem Entoderma zwar wohl 
sicher vermuthet werclen darf, aber noch nicht nachgewiesen ist, eine Hauptrolle.” 
On the other hand, Koch considers the axis as probably ectodermic in its origin. 
This author, while admitting the so-called axis of a certain division of the Gorgonacea 
(Pseudaxonia) to be mesodermic, has given very strong reasons for the belief that the 
true axis of many Alcyonaria is secreted by a layer of epithelial cells directly derived 
from the ectoderm. Kolliker himself observed that in some of the Pennatulida 
( Pteroides , Virguloria ) the axis is surrounded by a distinct epithelial layer, and 
Koch has shown that this is the case not only in other Pennatulacea, but also in 
those Gorgonida, which possess a true axis. Koch’s observations are conclusive 
that this epithelial layer, in the fixed Gorgonians, consists of invaginated ectoderm 
cells which secrete the axis as a cuticular structure. This “ axis-epithelium ” of the 
fixed Gorgonians is identical in structure with that of the Pennatulids, and the latter 
is believed by Koch, though from analogy only, to be also ectodermic, its original 
connexion with the exterior having been lost. 
In the face of such conflicting views as to the nature of the axis, it is impossible to 
determine its real relation to the peduncular septa and the septum transversale. 
Without definite knowledge on this point, it is clearly premature to frame any definite 
hypothesis as to the significance of the peduncular septum of Renilla, and the solution 
of this problem can only be found by studying the embryology of the axis-bearing 
Pennatulids. 
