370 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



pensatrix. Finally, the compensation sac may not end in the aper- 

 ture at all, but may open exteriorly by a special pore, the ascopore. 

 The form of the operculum is therefore in most cases identical 

 with that of the aperture, but the latter on fossil forms is not always 



Fig. 11. — Diagrammatic drawing of a Cheilostomatous bryozoan belonging to the sub- 

 order Ascophora, showing the ovicell (O), the ornamental calcareous frontal wall with 

 a spear-shaped avicularium and the form of the operculum with its posterior portion 

 (p), which leads into the compensation sac (c), the anterior portion (a) closing the 

 orifice of the polypide (P). The hinge or cardelle upon which the operculum operates 

 is shown at o and the communication pores or septulae, between the zooecia, at s. 



visible exteriorly, for it may be hidden by excessive calcification of 

 the frontal or by exterior organs such as the avicularia. The only 

 safe means of determining the true form of the aperture is the ex- 



12 13 



Fig. 12. — Opercula. Sketches showing the operculum in different genera of 



Cheilostomata. 



1—3. Anasca, no compensation sac (1, Thalamoporella ; 2,- Steganoporella ; 

 3, Aspidostoma) . 



4-13. Ascophora, illustrating the variations in form of the anterior portion 

 of the operculum (anter) through which the polypide emerged and the 

 posterior part (poster) by which water was introduced into the com- 

 pensatrix. (4, Trypostega; 5, Triphyllozoon ; 6, Sntittinaj 7, Holopo- 

 rella; 8, Stichoporina ; 9, Bipora; 10, Peristomella ; 11, Schismopora ; 

 12, Rhynchozoon j 13, Schizopodrella.) 



amination of the interior of the zooecium obtained by abrasion of the 

 basal surface. This preparation is easily made by mounting the 

 fragment to be studied in hard Canada balsam on a glass slip, cellu- 



