368 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



within special cavities of incubation which, when visible, are called 

 ovicells. A large number of species of Cheilostomata show no ovi- 

 cells and nothing on the exterior reveals their mode of reproduc- 

 tion. Some are oviparous and expel their eggs by an intertentacular 

 organ, but most of this order have some visible ovicell. An ovicell 

 of a particular form and position usually characterizes all of the 

 genera of a family, and it is of course an invariable rule that all 

 the species of a genus should bear the same kind of ovicell. In addi- 

 tion to the position of the ovicell, the relationship of the operculum 

 to the ovicell is also quite important. Its various methods of opera- 

 tion are illustrated in the accompanying diagram, which shows 

 sketches of the more important types of structure. (Text fig. 9.) 

 A section passing lengthwise through the zooecia or individual cells 

 is necessary to determine the nature of the ovicell as well as the gen- 

 eral structure. This section requires much care, as the specimen must 

 be mounted on edge and the abrasion must follow a definite row of 

 cells. By the use of small wire nippers it is easy to trim the speci- 

 men to just the right form, then by mounting it in hardened balsam 

 between two small bits of wood (fragments of a match serve excel- 

 lently) to hold it on edge, the abrasion can be continued until the 

 desired section is obtained. Actual dissection of the specimens with 

 a fine needle under the microscope is often necessary, especially to 

 determine the nature of the ovicell. 



Hydrostatic function. — The discovery of the zooecial hydrostatic 

 function by Jullien in 1888 explained many manifestations of the 

 bryozoan which for a long time had remained absolutely unknown. 

 This function of the extrusion of the polypi de is so important that 

 the two suborders of the Cheilostomata, the Anasca and Ascophora, 

 are based upon it. In the suborder Anasca the so-called compensa- 

 tion sac is wanting and the polypide is extruded from the zooecium 

 through the depression of the chitinous frontal wall by parietal 

 muscles. This feature, as well as the general anatomy of the poly- 

 pide in this order, is illustrated in text figure 10. In the Ascophora 

 the polypide can emerge from the zooecium only if an equal vol- 

 ume of water is introduced to compensate for the extrusion. For 

 this purpose the compensation sac (text fig. 11) or compensatrix 

 is placed beneath the dorsal under the larger part of the zooecial 

 length and communicates with the aperture. At the moment of ex- 

 trusion of the polypide, muscles attached to the compensation sac 

 contract, thus enlarging the sac, and the operculum in opening for 

 the extrusion of the polypide frees its orifice. A minute drop of 

 water then penetrates into the sac, thus compensating for the poly- 

 pide. The entrance of the water into the compensation sac is thus 

 the hydrostatic function and it is exercised in many ways which 



