640 



BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



The orifice or apertura. The orifice of the zooecial tubes is always open. 

 Figure 198 F represents the relations of the tentacular sheath and of the walls of 

 the zooecia in their superior or distal region. 1 It is therefore by tnrgescence that 

 the polypide may extrude or retract its tentacles. 



When the polypide dies from disease or accident, a calcareous lamella im- 

 mediately closes the tube. This lamella is superficial and finely perforated (fig. 



FIG. 199. Tubes with 

 facets. 



Loboscoecia (Meli- 

 ceritites ) aemicln IIXH 

 Michelin from the 

 Cenomanian at Le 

 Mans, Prance, X 14. 

 Surface of branch 

 showing zooecia with 

 facets. 



x25 



FIG. 200. Tubes with rhomboidal orifice. 



An ovicelled specimen of Homocosolen gam- 

 blei Gregory, 1909, X 25, from the Turonian 

 at St. Calais, France, illustrating rhomboidal 

 form of apertures. 



198 G) and varies much in form and position. It is sometimes perforated at the 

 middle (fig. 198 H) or surmounted by a projecting tubule (fig. 198 I). Again it 

 may be deep and invisible ; it is formed then about the point where the zoarial tube 

 rises free from the zoarium (fig. 198 J, K). It is sometimes double (fig. 198 L). 2 



Diaphragms. The diaphragms are rare and accidental in the club-shaped 

 ttilies, but are abundant and specific in the cylindrical; they are sometimes formed 

 in ijoth the accessory and the adventitious tubes. Their physiologic function is 

 unknown. 



Like the lamellae of closure they are entire or perforated; the perforation is 

 central or excentric. Figure 201 A, B will be useful in the interpretation of thin 

 sections. In our American Tertiary formations species with diaphragms are rare. 



1 1900. Calvet, Histoire naturelle des Bryozoaires ectoproctes, p. 180, fig. 24. 



- 1S84. Waters, Closure on the Cyclostomatous Bryozoa, Journal Linnean Society, p. 400, 404, pi. 17 



