CLASS I BRYOZOA 315 



walled sac {zocedum), and possesses typically a freely suspended alimentary canal 

 with mouth and anns. Mouth surrounded by a crown of hollow, slender, ciliated 

 tentacles arranged in the form of a circle or crescent. Usually hermaphroditic. 



Tlie Biyozoa resemble certain Corals (Tabulata) and Hydrozoans in their 

 external configuration, but differ from tbem radically in the possession of a distinct 

 body cavity, a closed alimentary canal, a highly developed nervous System, and 

 delicate respiratory tentacles siirroimding tbe moutb. With the exception of the 

 solitary geniis Loxosoma, all Bryozoans live associated in colonies or zoaria, of greater or 

 less extent, and of either calcareoiis, corneous or membranaceous composition. These 

 colonies, which are formed by frequently repeated gemmation, present a multitudinous 

 variety of form, habit and structure. Sometimes they grow into plant-like tufts, 

 composed of a series of cells variously linked together ; very commonly they spread 

 over Shells and other foreign bodies, forming delicate interwoven threads, crusts of 

 exquisite pattern, or hemispherical, globular or nodular masses of considerable size ; 

 often they rise into branching stems, and fronds of varying width ; and at other times 

 the cell-bearing branches form most regulär and beautiful open-meshed lace-work. 



Each zooid or polypide is enclosed in a separate Chamber (zooscium) of either 

 utricular or more or less tubulär form. Occasionally the zooecia are quite distinct 

 from their neighboui's ; more commonly, however, intercommunication is effected, 

 either by means of niinute " connecting foramina " piercing the Chamber walls, or 

 by a common canal to which all the zooids are attached. A true coenenchyma, 

 such as is found among the Coelenterates, never occurs, and coenenchymal gemmation 

 is accordingly unknown ; but a somewhat similar " vesicular tissue " not infrequently 

 occupies the interzooecial spaces which have resulted from the erection of the zocecial 

 tubes. 



Such vesicular tissue occurs constantly in the Fistuliporidae and Cystodictyonidae, and in 

 the latter the primary, or even the prostrate cells, are not entirely contiguous. The upper 

 walls of the vesicles, at least, are abundantly perforated ; and when with increasing age the 

 vesicles beconie fiUed with a secondary deposit, these pores are not obliterated, but continue to 

 pass through such deposits in the form of minute vertical tubes. Precisely the same kind of 

 tissue occurs in other Bryozoans, notably among adult colonies of certain Fenestellidae, in 

 which the expanded base of the colony is largely vesicular, and the fenestrules and spaces 

 between the carinae of the branches are filled with vesicles for some distance up. The real 

 purpose of this tissue is to support the zooecia and to strengthen the zoarium. 



However diverse the external aspect of the composite structure, the small animals 

 themselves conform to a simple and quite definite type. Briefly, the soft parts consist 

 of an alimentary canal, in which three distinct regions, an Oesophagus, stomach and 

 intestine, are recognisable. This is enclosed in a sac, and so beut upon itself that its 

 two extremities, or openings, approximate ; one of them, the oral, being either entirely 

 or partially surrounded by a row of slender, hollow and ciliated tentacles, which serve 

 for respiration and for sweeping food toward the mouth. In most cases the anal 

 opening is situated without the ring of tentacles (Ectoprocta), rarely within the same 

 (Entoprocta). Heart and vascular system are wanting, but a nervous ganglion, sending 



Nunierous papers on Mesozoic and Cenozoic Bryozoa in the Bull. See. Gi^ol., France, 1897-1910. — 

 Ulrich, E. 0., Eocene Bryozoa. Eocene volume, Md. Geol. Surv., 1901.— Ulrich, E. 0. and Bassler, 

 R. S., Miocene Bryozoa. Miocene volume, Md. Geol. Surv., 190\.— Ulrich, E. 0. and Bassler, R. S., 

 Revision of tlie Paleozoic Bryozoa. Smith. Mise. Coli., vols. xlv.-xlvii., 190i.—Nickles, J. M. and 

 Bassler, R. S., Synopsis of American fossil Bryozoa. Bull. 173, U.S. Geol. Surv., 1901. (Contains 

 a list of all bryozoan literature and a bibliograpliy of fossil forms.) — Bassler, R. S., Bryozoan Fauna 

 of the Rochester Shale. Bull. 292, U.S. Geol. Surv., 1906.— Gregor]/, J. W., Cat. Cretaceous 

 Bryozoa in British Museum, 2 vols., 1899 and 1909.— Levinsen, G. M. R., Cheilostomatous 

 Bryozoa (Recent). Copenhagen, 1909.— Bassler, R. S., Early Paleozoic Bryozoa of the Baltic 

 Provinces. Bull. 77, U.S. Nat. Mus., 1911.— Ilennig, A., Gotlands Silurische Bryozoen. Arkiv 

 Zool., 1908, vol. iv. — Canu, F., Iconographie des bryozoaires fossiles de I'Argentine. Anal. Mus. 

 Nac. Buenos Ayres, 1909-11, ser. 3, vol. x.—Lee, O. W., British Carboniferous Trepostomata. 

 London, 1912. 



