116 GUIDE TO THE FOSSIL INVERTEBRATE ANIMALS. 



BRYOZOA OR POLYZOA. 



Gallery The fossil remains of the Bryozoa (moss-animals), or 

 Polyzoa, as they are often called, a group of which the 

 Centre- modern Sea-mats are familiar examples, are exhibited in 

 A^-A6 three cases in the middle of the gallery, adjoining the 

 Wall-case Brachiopoda, with which they are by some supposed to be 

 12. allied. These specimens are all British, and form an adequate 

 representation of these widely spread fossils. In Wall-case 

 12 are placed a few foreign specimens of particular interest, 

 as well as certain colonies too large to find room in the 

 British series proper. 



c/i 



EiG. 61. — Diagram of structure of a typical Bryozoan zooid, as seen 

 in a vertical cut down the middle. an, anus ; ap, aperture ; b.c, 

 body-cavity ; c.p, communication pore ; d, diaphragm ; ect, ectoderm ; 

 e7id, endoderm ; /, funiculi ; n, nerve-ganglion ; o, orifice ; ces, oeso- 

 phagus ; op, operculum ; r.in, retractor muscle ; st, stomach ; T, 

 tentacles ; t.s, tentacle sheath. 



Though often mistaken by wanderers on the sea-shore for 

 sea-weeds, the Bryozoa are really animals. They live in 

 either fresh or salt water, mostly in the latter, and with 

 one exception {Loxosoma), always form colonies, which are 

 generally fixed. A colony consists of a large number of 

 individuals (or zooids), each of w^hich is completely separated 

 from the rest and enclosed in a double-walled sac. The 

 digestive tube is U-shaped, the mouth and anus being placed 

 close together. A band of tentacles occurs around the mouth 

 in most forms, but in one group (Entoprocta) this surrounds 

 both the mouth and the anus (Fig. 61). These colonies 

 may spread in delicate gauze-like sheets over weeds, shells. 



