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 zooeeia in their superior or distal region. 1 It is therefore by turgescence 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 ) semiclauw 
Michelin from the 
Cenomanian at Le 
Mans, France, X 14. 
Surface of branch 
showing zooeeia with 
facets. 
Fig. 200. — Tubes with rhomboidal orifice. 
An ovicelled specimen of Homoeosolen 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 he deep and invisible ; it is formed then about the point where the zoarial tube 
rises free from the zoarium (fig. 198 J, Iv). It is sometimes double (fig. 198 L). 2 
Diaphragms . — The diaphragms are rare and accidental in the club-shaped 
t ahes, but are abundant and specific in the cylindrical; they are sometimes formed 
in both 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 exeentric. 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. 
2 1884. Waters, Closure on the Cyclostomatous Bryozoa, Journal Linnean Society, p. 400, 404, pi. 17 
