322 SIR SIDNEY F. HAEMER ON 
p. 314), in the form of a cylindrical tube, rather longer than the future joint, 
and open at both ends. An annular zone of the calcareous wall is then 
absorbed, at the middle of the chitinous tube, which is left as the sole con- 
nection between the distal part of the zocEcium and the parent-internode. 
It thus follows that the jointed zooecium partakes in the formation of two 
internodes ; the greater part of it usuallj' lying in the proximal end of the 
■daughter-internode, while the part on the proximal side of the chitinous 
joint is immersed in the parent-internode. The polypide commonly passes 
through the jointed region of the zocEcium, not only in the early stage, before 
the absorption of the calcareous annulus, but through the chitinous tube even 
after the completion of the joint, and in fact throughout the whole period of 
its own life. The proximal segment of the jointed zooecium has often been 
described as a " special chamber " of the parent-internode, by authors who 
have not fully appreciated its morphological significance. 
In certain cases, as in species of Scrupocellaria and in Poricellaria {Diplo- 
didymia), further strength is afforded to the joint by the development of 
additional tubes of chitin, each formed on the inner side of its predecessor 
and somewhat longer than it. A considerable number of these tubes may be 
formed, and the whole chitinous complex thus acquires a considerable thick- 
ness, showing at each end a series of rings of diminishing diameter in passing 
from the outer to the inner surface of the tube. This arrangement, which 
may be described as a system of tubes " en echelon," is indicated by 
Claparede (1870, Zeitschr. wiss. Zool. xxi. pi. 9. fig. 1 C), and his figure also 
shows the circular, transverse furrows of the annular thickenings of the 
calcareous wall with which the edges of the successive chitinous tubes are 
firmlv united. This arrangement, which is most noticeable in the more 
robust species of Scrupocellaria, appears to be a mechanical arrangement 
admirably adapted to give the greatest strength at the middle of the joint, 
and to allow the entire stem the flexibilitj' which is required in order to obviate 
fracture of the delicate branches. 
The bifurcation of a biserial branch takes place by the doubling of the 
number of zocecia, and generally in such a way that two successive, alter- 
nating zooecia of opposite side of the parent-internode are each followed by 
two distal successors, instead of by a single successor as in parts where the 
internode is merely elongating without dividing. For the purpose of com- 
parison I have adopted a uniform notation for the zooecia concerned in the 
bifurcation, as may be seen by reference to figs. 1-18. The more proximal 
of the two zocEcia which prepare the way for, the bifurcation is distinguished 
as A, and the more distal zooecium, on the other side of the branch, as B. 
C and D, on the outer sides of the arms of the Y-shaped bifurcation, may be 
regarded as the direct successors of A and B respectively ; and it will be 
seen that they agree closely with their predecessors in form, differing from 
them only by diverging from one another to an extent sufficient to allow 
