258 MOLLUSCOIDEA BRYOZOA SUB-KINGDOM v 



walled sac (zocecium), and possesses typically a freely suspended alimentary canal 

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

 arranged in the form of a circle or crescent. Usually hermaphrodite. 



The Bryozoa, or Polyzoa, most nearly resemble certain Corals (Talulata) and 

 Hydrozoans in their -external configuration, but differ from them radically in the 

 possession of a closed alimentary canal, a highly developed nervous system, and 

 delicate respiratory tentacles surrounding the mouth. With the exception of the 

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

 lesser 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-bearing branches form most regular and beautiful open-meshed lace-work. 



Each individual is enclosed in a separate chamber (zocedum) of either utricular or 

 more or less tubular form. Occasionally the zocecia are quite distinct from their 

 neighbours ; more commonly, however, intercommunication is effected, either by 

 means of minute " 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 Cystodidyonidae, 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 individuals 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 zocecia and to strengthen the'zoarium. 



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

 themselves conform to a simple and quite definite type. Briefly, the animal consists 

 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 

 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 

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



Waters, W. A., Numerous papers on Tertiary and Recent Bryozoa in Ann. and Mag. Nat. Hist., 



and Quar. Journ. Geol. Soc., 1879-92. 



Hincks, T., History of the British Marine Polyzoa, 2 vols., 1880. 

 Vine, Q. R., Reports on fossil Polyzoa (British Assoc. Reports), 1881-85. 

 Ulrich, E. 0., American Palaeozoic Bryozoa (Journ. Cincinnati Soc. Nat. Hist., V.-VII. ), 



1882-84. 



Busk, G., Report on Polyzoa (Sci. Results Challenger Exped., Zoology, vols. X. and XVII.), 1884-86, 

 Hall, /., Lower Helderberg, Corniferous, and Hamilton Bryozoa (Palaeont. N.Y., vol. VI.), 1886. 

 Ulrich, E. 0., Contributions to American Palaeontology, vol. I. Cincinnati, 1886. 

 Pergens, K, Revision des Bryozoaires du Cretace, etc. (Bull. Soc. Beige Geol., III. -VI.), 1889-92. 

 Ulrich, E. 0., Palaeozoic Bryozoa (Geol. Survey Illinois, vol. VIII.). 1890. 

 Ulrich, E. 0., Lower Silurian Bryozoa (Geol. Survey Minnesota, Final Report, vol. III.), 1892. 



