800 
DR, E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
not exist so far as known in the simple young of other Alcyonaria (cf. Leptogorgici, 
p. 768). It is hard to avoid the conclusion that these structures are intimately con¬ 
nected with the formation of the peculiar internal axis in those Pennatulids (including 
all but Renilla ) which possess such a structure. In Renilla reniformis the septum 
transversale alone is developed (as the peduncular septum), but in Renilla amethy- 
stina, as described by Kolliker and Eisen, four partitions appear in the anterior part 
of the peduncle (see p. 770), which appear to be homologous with the four peduncular 
septa by which the axis is suspended in the axis-bearing Pennatulids. If, then, the 
latter forms and Renilla have independently arisen from the Archiptilum, which 
possesses no axis, it is impossible to account for the presence of the four peduncular 
septa in some species of Renilla. Whereas, if Renilla is descended from an axis¬ 
bearing form resembling Bathyptilum the occasional appearance of four peduncular 
septa presents no difficulty (compare § 9). 
As Kolliker has shown, the lateral pinnae ( Blatter ) of the Renniformes are 
probably derived from simple lateral buds by the appearance of a series of dorsal buds 
upon the latter :— 
“ In der That lehren die Pennatuliden mit Blattern, dass jedes Biatt anfanglich nur 
aus wenigen, wahrscheinlich urspriinglich nur aus Einem Polyp besteht und dass die 
tibrigen Indivicluen nach und nach an der Dorsalseite desselben aus Him hervorbilden, 
was theils durch Tlieilungen , theils durch Knospenbildungen aus ihm geschieht ” 
(‘ Pennatuliden,’ p. 430). 
At first thought it might seem probable that this dorsal series of buds is represented 
in Renilla by the series of dorsal zooids which always appear on the upper median line 
of the sexual polyps. But upon examination we find that the axes of the zooids are 
differently placed from those of the polyps. The ventral chambers of the former face 
inwards (towards the centre of the disc), whereas those of the polyps in the pinnae of 
the Renniformes face backwards; it seems therefore improbable that they can 
correspond. As we shall see in a following section, it is doubtful whether the zooids 
of Renilla are homologous with (i.e., directly descended from) sexual polyps. The 
representatives of the dorsal polyps of the pinnae, if present at all in Renilla, are 
rather to be sought in those lateral buds of Renilla which do not arise directly upon 
the body of the axial polyp. 
Summary. 
The development of the colony in Renilla indicates its ancestral origin from a form 
resembling the Batliyptilece from which have also been derived along different lines ol 
descent the Rennatulece on the one hand, and the Kopliobelemnoniece and Veretdlidce on 
the other. In the course of this transformation an axis has probably been lost, the 
only indication of it at present being the persistence of the septum transversale 
(peduncular septum) and in some species of the four peduncular septa. No decisive 
