Dr Harmer on the Structure and Classification, etc. 11 



On the Structure and Classification of the Cheilostomatous Polyzoa. 

 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 Siplionicytara 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 continuous 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 , Catenicella alata 

 and C. 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 lcs Biyozoaires 

 Cheilostomiens Monodermies," Bull. Soc. Zool. France, xm. 1888, p. 67. 



3 "Observations anatoiniques sur les Catenicelles," Mem. Soc. Zool. France, i. 

 1888, p. 274. 



