374 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



They usually have a straight proximal edge, which works against a 

 calcareous bar, or, when this is not complete, from two teeth. In 

 the fossil forms and in many dead specimens of recent species the 

 mandible has been lost, but its position is clearly indicated on the 

 porelike spaces left by the avicularia in well-preserved specimens. 



The vibracula (text fig. 15) are modified zooecia, similar to the 

 avicularia, but differing in the occurrence of a long cilium or seta in 

 place of the mandible. The porelike excavation it leaves in the fossil 

 forms does not show the variation of structure observed in the 

 avicularia. 



Classification. — From the foregoing discussion it will be noted that 

 more factors enter into the determination of a cheilostomatous bryo- 

 zoan than in those of any other order. First the presence or absence 

 of a compensation sac must be learned in order to place the species 

 in its proper suborder ( Anasca or Ascophora) . Then the relationship 

 between the operculum and the ovicell and, again, between the oper- 

 culum and the compensatrix, the position of the ovicell, the form of 

 the aperture, the nature of the frontal wall, which may be chitinous 

 or, when calcareous, may be smooth (olocyst), punctate (tremocyst), 

 or radiately ribbed (pleurocyst) , the occurrence of dietellae and sep- 

 tulae, and of avicularia and vibracula, as well as other more detailed 

 structural features which have not been discussed in this article, are 

 to be observed in turn. The proper description and illustration of a 

 species of Cheilostomata is a considerable task in itself, which can not 

 be accomplished simply by publishing a diagrammatic figure of the 

 zooecial surface characters. 



Formerly the classification of the Cheilostomata was based on 

 purely zoarial features, but in the latter half of the nineteenth cen- 

 tury the zooecial characters were more closely studied, especially by 

 Busk, 31 D'Orbigny, 32 Smitt, 33 and Hincks. 34 The latter author con- 

 sidered especially the form of the aperture — in other words, only the 

 hydrostatic system — but Jullien 35 in various publications emphasized 

 the more important characters for consideration. The microscopic 

 anatomy of the polypide in the Cheilostomata is described and illus- 

 trated in detail in Calvet's important contribution in 1900. 30 Various 



31 Busk George. Catalogue of Marine Polyzoa in Collection British Museum. Cheilo- 

 stomata (1852) ; Polyzoa collected hy II. M. S. Challenger, Pt. 1. Cheilostomata, Vol X, 

 Pt. XXX (1884), and various articles entitled " Zoophytology in the Quarterly Journal 

 Microscopical Science from 1855 to 1867. , 



3 - D'Orbigny, Alcide. Paleontologie francais, Terrain Cretace, Vol. V, 1852. 



33 Smitt, F. A. Kritisk fortecknig ofver Skandinaviens Hafsbryozoer. Ofversigt af 

 Kongl. Vetenskaps-Akademiens Forhandlingar, vols. 22 to 24 (1865-1867) and vol. 28 

 (1871). 



34 Hincks, Thomas. British Marine Polyzoa (1880). 



36 Jullien, Jules. Mission Scientifique du Cap Horn, 1882, 1883, Vol. VI, Zoologie, 

 Bryozonires (1888). 



38 Calvet, L. Contributions a l'historle des bryozoaires ectoproctes marins. Travaux de 

 l'institut Zoologie de l'Universite de Montpellier, new ser., Mem. No. S, 1900. 



