MOLLUSCA : POLYZOA, 



373 



zooids are called " polypides ; " and the little chambers in 

 which each is contained are called the " cells," or " zocecia." 



It will be seen, therefore, that the term polypite is restricted 

 to the zooid of a compound Hydrozoon, or to the entire hydro- 

 soma of a simple member of the class. The term polype is 

 applied to a simple Actinozoon, or to the zooids of a compound 

 actinosoma. Lastly, the term polypide is exclusively employed 

 to designate the zooid of one of the Polyzoa. 



The construction of a typical polypide of a Polyzoon is thus 

 described by Professor Allman (fig. 198, 2) : 



"Let us imagine an alimentary canal, consisting of oeso- 



Fig. 198. Morphology of Polyzoa. i. Portion of the coenoecium of Flustra truncata, 

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

 rounded by tentacles ; b Alimentary canal ; c Anus ; d Nervous ganglion ; e Invest- 

 ing sac (ectocyst); y~Testis; /' Ovary; Retractor muscle. 3. Bird's-head pro- 

 cess, or "avicularium," of a Polyzoon. 



phagus, stomach, and intestine, to.be furnished at its origin 

 with long ciliated tentacula, and to have a single nervous gan- 

 glion placed upon one side of the oesophagus. Let us now 

 suppose this canal to be bent back upon itself towards the 

 side of the ganglion, so as to approximate the termination to 

 the origin. Let us further imagine the digestive tube thus 

 constituted to be suspended in a fluid contained in a mem- 

 branous sac with two openings, one for the mouth and the 

 other for the vent, the tentacula alone being external to the 

 sac. Let us still further suppose the alimentary tube, by means 

 of a system of muscles, to admit of being retracted or pro- 



