60 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
mucro, by the avicularia, or at the base of the peristomie. The only practical 
means of recognizing the apertura is the examination of the interior of the zooecium, 
obtained by abrasion of the basal wall. Each time that it is possible one should 
not hesitate to make this preparation. If should be noted further that many of the 
apertures are oblique and that their true form can be observed or drawn only by a 
suitable tilting of the specimen, placing the plane of the apertura parallel to the 
objective. In some rare species the operculum does not correspond exactly to the 
apertura ; in such cases it is evident that for the paleontologist the problem can not 
be solved. 
According to Smitt, Hincks, and Jullien, the form of the operculum character- 
izes the families. Some results obtained by the study of embryology do not permit 
this theory to be held any longer. In the same family characterized by the same 
larva the entrance of the water into the compensatrix may occur in diverse manners. 
It is indubitable, however, that the form of the apertura is an excellent generic 
character. 
Ascopore . — -The compensatrix does not always open into the apertura, but it 
may open exteriorly on the frontal by an ascopore (micropore). The operculum 
is then semilunar with a straight proximal border (figs. 12, •/, K) . 
Hypostege .- — In the Anasca the hydrostatic system is external, as it is formed 
of a cavity situated between the cryptocj^st and the ectocyst. The cryptocyst is that 
part of the skeleton immediately in contact with the endocyst. The parietal muscles 
are attached to the cryptocvst when the latter is flexible (Flustridae) ; they are 
attached to the ectocyst when it is calcified (Onychocellidae, Steganoporellidae, 
etc.). Here the sea water can not penetrate into the hypostege, for there is no 
operculum, but a simple opercular valve intimately joined to the ectocyst. But 
certainly there is a liquid in the hypostege ; according as it is introduced or expelled, 
it serves as a compensation to the polypide. 
All the zooecial hyposteges evidently communicate with each other and form a 
zoarial hydrostatic system. The simultaneous extrusion of all the 'polvpides of the 
same zoarium is therefore impossible; this is a notable inferiority in the internal 
hydrostatic system of the Ascophora. 
ZoaHal hydrostatic system . — Calvet designated as the hypostege the space 
included between the ectocyst and the cryptocyst. We believe that it is necessary 
to make a distinction between this zoarial hypostege and the zooecial hypostege 
described by Jullien and quite visible on a very large number of Anasca. It is 
evident that all the hyposteges communicate with each other under the ectocyst. 
In many species (Lunulariidae) with a chitinized and thick ectocyst, this 
zoarial hydrostatic system is quite important. 1 It permits adaptation to diverse 
aquatic conditions and functions according to the principle of Archimedes. 
The species which creep on algae have small tuberosities. The flexible and 
extensile ectocyst alone is fastened on the algae and the small tuberosities glide 
1 1915. Canu, Le systeme bydrostatique zoarial des Bryozoaires cbeilostomes, Bulletin Soei£te Geo- 
logique France, ser. 4, vol. 15, p. 21. 
