BRTOZOA 245 



SUBPHYLUM 4. BRYOZOA.* (POLYZOA.) 



Minute and mostly colonial animals which are attached to rocks, 

 plants, and other objects in the sea or fresh water. The colony is usu- 

 ally made up of hundreds or thousands of individuals which have arisen 

 from one another by a process of budding, and is often mosslike in 

 appearance, whence the name of the group. The Loxosomidae are the only 

 non-colonial family. The individual members of a colony are called the 

 zooids : they are more or less cylindrical in form and are af ten polymorphic 

 in structure. The outer wall of the zooid is in most cases a thick cuticula 

 secreted by a hypodermal cell layer : it is often hardened by the presence 

 of calcium carbonate and forms a rigid case within which lie the soft parts 

 of the animal. This case, which is called the ectocyst or zooecium (Fig. 

 406,8), will often remain long after the death of the animal and the dis- 

 appearance of the soft part. In Pectinatella and some other forms the 

 body wall is fleshy or jelly-like. 



The soft parts of a zooid consist of the viscera and the tentacle- 

 sheath with the tentacles which constitute the anterior end of the body. 

 The tentacles are hollow and ciliated and are borne upon a prominent oval 

 or horseshoe-shaped ridge called the lophophore (Fig. 406,1). 



The body wall below the tentacles is highly flexible and in the 

 Ectoprocta these can be completely retracted within the zocecium. In the 

 center of the lophophore is the mouth and in the Entoprocta the anus 

 also: in the Ectoprocta the anus is situated just outside of it. The ten- 

 tacles are the only portion of the external surface of the Bryozoa that 

 is ciliated. 



The internal organs differ very much in the two great groups of the 

 Bryozoa and will be described when these are presented. 



Distribution and Habits. The majority of Bryozoa are marine, being 

 found from tide water to very great depths. Between tide lines and in 

 shallow water incrusting and creeping colonies which are attached to rocks, 

 shells, or seaweed are common, while in deeper water erect and branching 

 colonies are the more abundant. No Bryozoa are parasitic, although many 

 species live commensally with other animals or with plants. The group is a 

 very ancient one, occurring in the Cambrian and all subsequent formations. 



* See "Report upon the Invertebrate Animals of Vineyard Sound," etc., by A. B. 

 Verrill, Rep. U. S. Com. Fish., 1871-72, p. 292. "British Marine Polyzoa," by 

 Thomas Hincks, London, 1880. "Synopsis of North American Invertebrates, I. 

 Freshwater Bryozoa," by C. B. Davenport, Am. Nat, Vol. 33, p. 593, 1899. "Sponges 

 and Bryozoa of Sandusky Bay," by F. A. Landacre, The Ohio Naturalist, Vol. 1, p. 

 96, 1901. "The Bryozoa. Papers from the Harriman Alaska Expedition," by Alice 

 Robertson, Proc. Wash*. Acad., Vol. 2, p. 315, 1900. "The Freshwater Bryozoa of 

 the United States," by C. B. Davenport, Proc. U. S. Nat. Mus., Vol. 27, p. 211, 1904. 

 "The Bryozoa of the Woods Hole Region," by R. C. Osburn, Bull. Bur. Fish., Vol. 30, 

 1912. "The Bryozoa of Tortugas," by same, Pub. No. 182, Cam. Inst., Wash., 1914. 



