634 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



The ovicells on the fossil forms have not escaped observation as D'Orbigny, 

 Hagenow, Reuss, and Pocta have figured them, although these students did not 

 recognize their great value in classification. In 1897 1 Canu discovered the ovicell 

 of the Melicerititidae, and in 189!) 2 that of the Ceidae. In 1898 3 he published 

 some special variations and indicated the great necessity for the study of the 

 ovicell. In 1918 4 he published a summary of his new ideas on the classification 

 of the Cyclostomata with descriptions of a number of new genera, a work which 

 was preliminary to the present one where these ideas are more fully developed. 



GENERAL CONSIDERATION OF THE CYCLOSTOMATA. 

 THE TUBES. 



The zooecia of the cyclostomatous bryozoa are tubular.. Whatever may be the 

 zoarial form, each tube grows from a proximal tube by a special mode of gem- 

 mation (fig. 196 A, B). 



Considering the form of the tubes there are : 



(a) Club-shaped tubes or typical Cyclostomata. 



(&) Cylindrical tubes or Cyclostomata resembling the trepostomatous bryozoa 



(c) Tubes with facets. 



(d) Tubes with rhomboidal orifice. 



This division, however, is not an absolute one. Hollow zoaria (in the Cavaria 

 form of growth) often have shortened tubes the exact nature of which it is im- 

 possible to determine. In the zoarium of Lichenopora. the polypidian tubes are 

 club-shaped and the accessory tubes cylindrical. 



Considering their functions the tubes are classed as follows: 



(a) Polypidian tubes. 



(&) Accesory tubes (= aborted tubes of authors). 



(c) Adventitious tubes. 



The tubes are often grouped in longitudinal, transversal, or radial bundles 

 (fascicles) which may be monoserial or pluriserial. 



Calcification. The ectocyst is formed of two thin, flexible membranes between 

 which the calcareous skeleton is located (fig. 196 C) 5 ; the tubes are therefore 

 not strictly adjacent. The disappearance of the external membrane leaves a 

 space which, in thin sections, appears as a clear line. This intercellular space is 

 quite variable in size and is linear or vesicular. At the center of the agglomerate 

 zoaria the tubes are often adjacent by reduction, reabsorption or filling up; rarely 

 they are so in the entire zoarium (see Exochoecia). 



1 1897. Canu, Bryozoaires <Ju Turonien des Janieres, Bulletin Soci^tS Ggologique de France, ser. 3, vol. 

 25, p. 155. 



2 1890. Canu, Les Ovieelles des Ceidees, Bulletin Societe Ggologique de France, ser. 3, vol. 27, p. 326. 



3 1898. Canu, Etude sur les Ovieelles des Bryozoaires du Bathonien d'Occaignes, Bulletin Societe 

 Geologique de France, ser. 3, vol. 26, p. 259. 



4 1918. Canu. Les Ovieelles des Bryozoaires Cyclostomes, Bulletin Socie'tg Gfiologique de France, 

 ser. 4, vol. 16, pp. 324-335. 



B 1900. Calvet. Contributions a 1'histoire naturelie des Bryozoaires ectoproctes marins, Travaux In- 

 stitut Zoologique University Montpelller Mem., No. 8, p. 166. 



