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BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
part of the opercular and muscular manifestations for a long time remaining abso- 
lutely enigmatical. 
In the Ascophora the polypide can emerge from the zooecium only if an equal 
volume of water compensates this extrusion. For this purpose a sac called the 
compensatrix is placed under the dorsal. This sac communicates with the apertura 
and occupies a part of the zooecial length; to it varietal muscles are attached. At 
the moment of extrusion of the polypide, the muscles contracting, enlarge the com- 
pensatrix, the operculum in opening frees its orifice and a minute drop of water 
penetrates into it, thus compensating for the polypide. 
The entrance of the water into the compensatrix is a function which is exercised 
in many ways, indicated by the frontal and by the operculum. 
0 perculum.— The operculum is a quite small chitinized organ, closing at the 
same time the apertura and the compensatrix. It bears only a trace of occlusor 
muscles (=retractor), because the polypide itself in emerging pushes open the 
operculum. 
The anterior part, or anter, closes the orifice of extrusion of the polypide, or 
porta; the posterior part, or poster ; closes the orifice of the compensatrix. or vanna. 
The principal cases of this function of the introduction of the water into the 
compensatrix are mentioned below. 
If the orifice of the compensatrix be very small, the latter is closed by a small 
valve of the operculum, to which a small cleft or fissure of the apertura, the rimule, 
corresponds (Schizoporellae) . This operculum is chitinized and rigid; it moves 
on two condyles and bears the marks of two feeble occlusor muscular bundles 
(figs. 12, A-C). 
If the orifice of the compensatrix be large, it is closed by a large valve of the 
operculum, to which corresponds a large vanna, whose size is inversely propor- 
tional to the length of the compensatrix. The hydrostatic effort being considerable, 
the operculum is chitinized and rigid and sways on two cardelles; it bears two 
lateral bands for the attachment of the powerful occlusor muscles ( Hippoporina ) 
(figs. 12, D-F). 
When the compensatrix is attached to the operculum the latter is thin, fragile, 
semilunar, with a proximal border straight or convex. It does not always exactly 
cover the apertura, whose form is nevertheless almost identical but with a proximal 
border always concave. The orifice not covered by the operculum is that of the 
compensatrix; it is protected by a lyrula or a mucro , and, according to the hydro- 
static pressure, it moves on two condyles ( Smittina ) (figs. 12, G-I). 
In order to diminish the arrival of the water there is often a small peristomial 
canal, or pseuclorimule , or rimule- spir amen, or a spiramen opening into the 
peristomie. 
The form of the operculum is therefore identical with that of the apertura ; but 
the latter on the fossils is not always visible exteriorly, for it may be hidden by the 
1 Levinsen in 1909, in his Systematic and Morphological Studies on the Cheilostomatous Bryozoa, pp. 
37-16, published a very complete study on the operculum. 
