Dr Harmer on the Structure and Glassification, etc. 11 
On the Structure and Glassification of the Gheilostomatous Poly 20 a. 
By Sidney F. Harmer, Sc.D., King’s College. 
[Read 29 October 1900.] 
The first part of Busk’s Report on the “Challenger” Polyzoa 1 
contains an account of a cavity, separated from the general body- 
cavity by a delicate membrane, in the zooecia of Siphonicytara and 
Gephyrophora, two genera of Cheilostomatous Polyzoa. In the 
former the cavity opens to the exterior by the so-called “ median 
pore.” Although nothing is said of any external opening in the 
latter, the important suggestion is made “ that this division of the 
zooecial cavity into two compartments by a flexible membranous 
diaphragm will be found pretty generally in all zooecia of which 
the wall is wholly rigid, and that it is intended for the purpose of 
allowing the compression of the perigastric cavity necessary to 
effect the protrusion of the polypide.” The exact details of the 
mechanism are not further discussed ; and Busk states, on p. 175 
of his Report, that the nature of median pores is “at present 
wholly unknown.” 
Four years later Jullien published an important note 2 on the 
same subject ; without, however, referring to Busk’s statements. 
The results arrived at were, (i) the operculum, in certain Cheilo- 
stomes which have a rigid body-wall, is not coDtinuous proximally 
with the adjacent calcareous wall of the zooecium ; (ii) its 
proximal border does not form the hinge-line, which is situated 
more distally ; (iii) the interval between the proximal border of 
the operculum and the calcareous wall is the opening of a chamber 
which lies in the body-cavity and can be dilated by the admission 
of water in order to compensate for the protrusion of the polypide 
from its zooecium ; the chamber opens widely to the exterior when 
the operculum is open. This “ compensation-sac” was described 
by Jullien in greater detail in another paper 3 , Gatenicella alata 
and G. ventricosa being the species principally studied ; but its 
mode of action and its full significance were not thoroughly 
appreciated in this or in any of Jullien’s later papers. 
1 Part xxx., 1884, pp. 101, 168. 
2 “ Sur la sortie et la rentr^e du polypide dans les zooecies dans les Bryozoaires 
Cheilostomiens Monodermies,” Bull. Soc. Zool. France, xm. 1888, p. 67. 
3 “Observations anatomiques sur les Cat^nicelles, ” Mem. Soc. Zool. France, 1 . 
1888, p. 274. 
