CHAPTER XV. 



PHYLUM POLYZOA.* 



Small animals usually united together in colonies. With ciliated 

 tentacular crown^ \J-shaped alimentary canal, and simple ganglion. 

 With coelom, but without vascular system. Asexual reproduction by 

 budding always found. 



The Polyzoa (J. V. Thompson) or Bryozoa (moss-like animals) as 

 they are sometimes called (Ehrenberg), are with rare exceptions 

 (Loxosoma) colonial in habit. The colonies, which may present a 

 superficial resemblance to colonies of Hydroids, may have a foliaceous 

 or dendritic appearance, or they may form crusts on the surface of 

 foreign objects. The erect forms may have a calcareous ectocyst, 

 and be rigid, in which case owing to their brittle character they 

 are not found between tide-marks ; or they are flexible. Flexibility 

 may coexist with a calcified ectocyst, but in this case there are 

 horny joints at intervals. In such jointed colonies the part between 

 any two joints is an inter node. 



The individual zooids of the colony (Fig. 440) are small polyp- 

 like organisms, and usually possess a horny, or coriaceous,^ frequently 

 calcareous, rarely gelatinous exoskeleton, which is really the cuticle 

 of the zooids, and is secreted by the ectoderm. This cuticular layer 

 is called the ectocyst, and the case formed by it is the cell. 



The soft part of the body-wall, which consists of ectoderm and 

 mesoderm, lies close beneath the ectocyst, and is called the endocyst. 

 The ectocyst and endocyst together constitute the zooecium ; while 



* T. Hincks, "A History of the British Marine Polyzoa," London, 1880, 

 2 vols. G. J. Allman, Monograph of Fresh-water Polyzoa, Kay Society, 1856. 

 G. Busk, "Report on the Polyzoa," Challenger Reports, vols. 10 and 17, 1884 

 and 1886. K. Kraepelin, "Die Deutschen Siisswasser-Bryozoen," Abh. Natur- 

 wiss. Ver. Hamburg, Bd. x., 1887, and Bd. xii., 1892. E. C. Jelly, Synonymic 

 Catalogue of the Recent Marine Polyzoa, London, 1889. S. F. Harmer, " On the 

 Nature of the Excretory Processes of Marine Polyzoa," Quart. J. Mic. Sci., 

 vol. 33 ; and ' ' On the Occurrence of Embryonic Fission in Cyclostomatous 

 Polyzoa," Ibid., vols. 34 and 39, 1893 and 1896. S. F. Harmer, "Polyzoa," in 

 the Cambridge Natural History, -London, 1896. * 



