CLASS I BRYOZOA 315 



walled sac (soo&cium), and possesses typically a freely suspended alimentary canal 

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

 tentacles o^rranged in the form of a circle or crescent. Usually hermaphroditic. 



The Biyozoa resemble certain Corals (Tabulata) and Hydrozoans in tlieir 

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

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

 delicate resj)iratory tentacles surrounding the mouth. With the exception of the 

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

 less extent, and of either calcareous, 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-jjearing branches form most regular and beautiful open-meshed lace-work. 



Each zooid or polypide is enclosed in a separate chamber (zooxium) of either 

 utricular or more or less tubular form. Occasionally the zooscia are quite distinct 

 from their neighbours ; more commonly, however, intercommimication is effected, 

 either by means of minute " connecting foramina " jjiercing 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 

 occuj)ies the interzocccial spaces which have resulted from the erection of the zooecial 

 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 become tilled 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 tlie carinae of the branches are filled witli vesicles for some distance up. The real 

 purpose of this tissue is to support the zocecia 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 bent upon itself that its 

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

 or jiartially 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 vascidar system are wanting, but a nervous ganglion, sending 



Numerous papers on Mesozoic and Cenozoic Bryozoa in the Bull. Soc. Geol., France, 1897-1910. — 

 Ulrkh, E. 0., Eocene Bryozoa. Eocene volume, Md. Geol. Surv., 1901. — Ulrich, E. 0. aud Sassier, 

 R. S., Miocene Bryozoa. Miocene volume, Md. Geol. Surv., 1904. — Ulrich, E. 0. and Bassler, R. S., 

 Revision of the Paleozoic Bryozoa. Smith. Misc. Coll., vols, xlv.-xlvii., 1904. — 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 bibliography of fossil forms.) — Bassler, R. S., Bryozoan Fauna 

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

 Bryozoa in British Museum, 2 vols., 1899 aud 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. — Hennig, A., Gotlands Silurische Bryozoen. Arkiv 

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

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

 London, 1912. 



