258 MOLLUSCOIDEA—BRYOZOA SUB-KINGDOM V 
walled sac (zowcium), and possesses typically a freely suspended alimentary canal 
with mouth and anus. Mouth surrounded by a crown of hollow, slender, ciliated 
tentacles arranged in the form of a circle or crescent. Usually hermaphrodite. 
The Bryozoa, or Polyzoa, most nearly resemble certain Corals (Tabulata) 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 (zowcium) 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 zodids 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 
interzocecial spaces which have resulted from the erection of the zocecial tubes. 
Such vesicular tissue occurs constantly in the Pistwliporidae 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 filled 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 Feneste/lidae, 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 (Hetoprocta), rarely within the same 
(Entoprocta). 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, G. R., Reports on fossil Polyzoa (British Assoc. Reports), 1881-85. 
Ulrich, E. O., 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, J., Lower Helderberg, Corniferous, and Hamilton Bryozoa (Palaeont. N.Y., vol. VI.), 1886. 
Ulrich, E. O., Contributions to American Palaeontology, vol. I. Cincinnati, 1886. 
Pergens, E., Revision des Bryozoaires du Cretacé, etc. (Bull. Soc. Belge Geol., IIT.-VI.), 1889-92. 
Ulrich, E. O., Palaeozoic Bryozoa (Geol. Survey Illinois, vol. VITI.), 1890. 
Ulrich, E. O., Lower Silurian Bryozoa (Geol. Survey Minnesota, Final Report, vol. III.), 1892. 
