POLYZOA. 407 



— with its digestive tube, tentacula, ganglion, muscles, generative 

 organs, circumambient fluid, and investing sacs — repeating itself 

 by gemmation, and thus producing one or more precisely similar 

 systems, holding a definite position relatively to one another, while 

 all continue organically united, and we shall then have the actual 

 condition presented by the polyzoa in their fully developed state." 



The vast majority of the Polyzoa are fixed, but this is not 

 universally the case. The two investing sacs of the ''ccenecium " 

 of a Polyzoon have been called by Dr. Allman the " endocyst " 

 and the " ectocyst." The " ectocyst," or external investment of 

 the coenecium, is usually a brown, pergamentaceous, probably chi- 

 tinous, but often highly calcareous membrane ; and it is by the 

 " ectocyst " that the cells are formed. In Crlstatella alone, of the 

 Polyzoa, there is no " ectocyst." 



In many cases the " ectocyst '"' is provided with peculiar 

 appendages, supposed to be weapons of offence and defence, or 

 organs of prehension called " avicularia " (Fig. 3). The avicula- 

 ria, or " bird's head process," differ a good deal in shape, but 

 consist essentially (to quote the language of Bush) " of a move- 

 able mandible and a cup furnished with a horny beak, with which 

 the point of the mandible is capable of being brought into oppo- 

 sition." In shape the avicularia often resemble the head of a bird. 

 They keep up a snapping movement, which continues after the 

 death of the general colony. The "endocyst" is always soft, 

 contractile, and membranous. 



The mouth, according to Nicholson, conducts by an oesopha- 

 gus into a dilated stomach. In some cases a pharynx may be 

 present, and in others there is in front of the stomach a muscular 

 proventriculus or gizzard. From the stomach proceeds the intes- 

 tine, which shortly turns forward to open by a distinct anus close 

 to the mouth. As the nervous ganglion is situated on that side 

 of the mouth towards which the intestine turns in order to reach 

 its termination, the intestine is said to have a " nerved flexure," 

 and this relation is constant throughout the entire class. 



Respiration in the Polyzoa appears to be carried on by the 

 ciliated tentacles, and by the " perigastric space," which is filled 

 with a clear fluid, containing solid particles in suspension. A kind 

 of circulation is kept up in this " perigastric fluid " by means of 



