320 SIR SIDNEY F. HARMEE ON 
IV. Methods of Bifurcation of the Colony. 
The " Cellularine " series of Cheilostomatous Polyzoa, which takes its 
name from the genus Cellularia (see Sect. II.), consists of numerous genera 
and species in which the zocBcium has a membranous frontal wall and the 
colony has an erect, branching habit. The character of the frontal wall 
places them in Levinsen's Sub-order Anasca (1909, p. 91). The Cellularine 
habit of growth is closely parallelled, however, in members of the Sub-order 
Ascophora, iu which the frontal wall is calcareous and a compensation-sac is 
present [cf. Harmer, 1902, ref. on p. 295). The genus Catenicella and its 
allies may be mentioned in this connection ; but none of the Ascophorous 
genera are here considered. 
The Anascous branching forms are usually placed, in systematic treatises, 
near the commencement of the Cheilostomatous series; and it has often been 
at least tacitly assumed that they represent a low stage in the evolution of 
the Cheilostomata. This view seems to me erroneous; and the highly 
evolved nature of this assemblage is indicated by the characters of their 
heterozooecia, a term introduced by Levinsen (Vid. Medd. Naturh. Foren. 
Copenhagen, 1902, p. 3) to include the avicularia and vibracula. The avi- 
■cularium reaches the summit of its development in such Cellularine genera 
as Bugula and Cornucopina, while the vibraculum is highly specialized in 
Caberea, belonging to the same assemblage. The assumption that the 
branching habit is in any sense primitive appears to me fundamentally 
wrong. It is no doubt true that the Cellularine species are less adapted for 
.preservation as fossils than the encrusting forms ; but, making every allowance 
for this consideration, the Palseontological evidence points to the encrusting 
habit as the more primitive ; and already in the Cretaceous Period, large 
numbers of encrusting Cheilostomes, of a primitive type in other respects, 
are known. 
Not only are there these reasons for viewing with suspicion the claims of 
the Cellulai-ine genera to be regarded as representing an earl}^ stage in 
evolution, but the study of their mode of branching leads readily to the 
conclusion that the erect colony is a lamina which has been more or less 
subdivided. It may be noticed that the majority of Cellularine species 
consist entirely of branches having two surfaces sharply differentiated. The 
basal surface of the branch shows merely the "backs" of the individual 
zocecia, all of which have their orifices on the opposite or fronial surface. 
There is thus no difficulty in regarding the typical Cellularine colony as a 
unilaminar sheet of zocecia, divided by more or less radial slits into narrow 
branches, which in the majority of species are built up of two longitudinal 
series of zocecia, alternating on the two sides of the branch. It might appear 
■logical to regard the biserial condition as representing the last stage in this 
process, and thus to suppose that when biserial and multiserial branches 
■occur in closely related species, the multiserial condition should in all cases 
