300 MANUAL OF ZOOLOGY. 



are in the majority of instances more or less highly charged 

 with carbonate of lime. 



The homomorphism, however, which subsists between the 

 Polyzoa and the Hydroida is shown most decisively not to be 

 a true affinity, when the structure of the individual zooids is 

 examined. The polypite of a Hydroid Zoophite, as we have 

 already seen, possesses no alimentary canal distinct from the 

 general cavity of the body ; there are no traces of a nervous 

 system, and the reproductive organs are in the form of exter- 

 nal processes of the body-wall. In the zooid of all the Polyzoa 

 (fig. 123, 2), on the other hand, there is a distinct alimentary 

 canal, completely shut off from the somatic cavity ; a nervous 

 system is present, and the reproductive organs are contained 

 within the body. 



The following are the more important differences in the ter- 

 minology employed to designate the various parts of the com- 

 pound growths of the Polyzoa and the Hydrozoa. In the 

 Hydroida the entire colony is called the " hydrosoma,' ; and its 

 investing layer, when present, is called the " polypary," or 

 " polypidom ;" whilst the individuals composing the hydrosoma 

 are called the " polypites," and the cups in which these are 

 in some cases contained are called " hydrothecae." In the 

 Polyzoa the entire colony or its entire dermal system is 

 called the " polyzoarium " or " ccencecium ; " the separate zooids 

 a.re called "polypides;" and the little chambers in which each 

 is contained are called the " cells." 



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 tercn. polypidt is exclusively employed 

 to designate the zooid of one of the Polyzoa. 



The construction of a typical polypide of a Polyzob'n is thus 

 described by Professor Allman (fig. 123, 2) : 



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

 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 



