644 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
D actylethrae . — The clactylethrae are club-shaped tubes without polypide, with 
the same diameter as the polypidian tubes; their orifice is polygonal and closed 
by a calcareous lamella very finely perforated (fig. 205). They resemble the 
tergopores in longitudinal sections but they differ in their calcareous closure. 
Exteriorly the clactylethrae resemble cancelli but differ from them in longitudinal 
A. Transverse thin sections, X 35, of two cancelli of TAchenopora holdsworthi Busk, 1875, 
showing spinules of the interior. (After Waters, 1894.) B. Tangential thin section of the dorsal, 
X 25, of Lichenopora grignonensis Milne Edwards, 1838. O. Longitudinal thin section, X 25, of 
Lichenopora goldfussi Iteuss, 1864. The cancelli are superposed and ramified. 
section, in their club-shaped cylindrical form, and in the absence of internal 
spinules. They differ from the tergopores in their aspect which is oblique and not 
at right angles, and in their calcareous closure. 
The physiological function of the clactylethrae is unknown. 
Cancelli . — The cancelli have been interpreted in many different ways, 1 but fol- 
lowing the principle of least change we have adopted the meaning held by the 
zoologists. They are the cylindrical tubes closed by a finely perforated calcareous 
lamella, which are garnished in the interior with numerous spinules. At the center 
of the zoaria the cancelli are completely cylindrical: on the margins their lower 
part is club-shaped (fig. 206). 
1 For their history see Gregory, Catalogue Cretaceous Bryozoa, vol. 1, p. 12 ; vol. 3, p. xx. 
B x 25 
Fig. 206. — Cancelli. 
