IQ2 MOLLUSCA. 



in a circle in the marine Polyzoa, but are disposed in the 

 shape of a horse-shoe in most of the fresh-water forms. The 

 digestive canal passes through the body-cavity, without open- 

 ing into it, and terminates in a distinct anus placed near the 

 mouth. The only other organs possessed by the polypide are 

 a nervous ganglion, and the organs of reproduction, each 

 zooid being hermaphrodite. 



Fig. 131. Morphology of Polyzoa. i. Portion < 

 magnified. 2. Diagram of a PolyzoOn (after Allma 

 by tentacles ; b Alimentary canal ; c Anus ; d Nei 

 cyst) ; f Testis ; f Ovary ; g Retractor muscle. 

 num,"ofaPolyzo5n. 



f the ccenoecium of Flustra tntncata, 

 i): a Region of the mouth surrounded 

 vous ganglion ; e Investing sac (ecto- 

 3. Bird's-head process, or "avicula- 



In order, then, to arrive at a clear conception of the struc- 

 ture of a Polyzbon, we have simply to imagine that such a 

 polypide as above described should have the power of repeat- 

 ing itself by gemmation, " thus producing one or more pre- 

 cisely similar systems, holding a definite position relatively to 

 one another, while all continue organically united." 



The only element of the Polyzoa with which the palaeonto- 

 logist is concerned is the external investment of the colony 

 the " coencecium " or " polyzoarium." This is formed by the 

 combined ectocysts of the various polypides, and it varies 

 greatly both in form and actual composition. In form, it may 

 be plant-like, rooted at one point, and rising into foliaceous 

 expansions or arborescent growths ; or it may spread over 

 some foreign object as a continuous crust. In consistence, it 

 may be fleshy, horny, sub-calcareous, or completely calcareous ; 



