192 



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. 



J 



Fig. 131. Morphology of Polyzoa. i. Portion of the coenpecium of Flustra truncata, 

 magnified. 2. Diagram of a Polyzoon (after Allman) : a Region of the mouth surrounded 

 by tentacles ; b Alimentary canal ; c Anus ; d Nervous ganglion ; e Investing sac (ecto- 

 cyst) ; f Testis ; f Ovary; Retractor muscle. 3. Bird's-head process, or "avicula- 

 rium," of a Polyzoon. 



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

 ture of a Polyzoon, 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 " ccenoecium " 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 ; 



