POLYZOA. 



intercommunication, comparable in kind perhaps, though not 

 in degree, to what obtains in many of the compound Ascidi- 

 ans. That this community exists is proved by the otherwise 

 inexplicable circumstance that the polyzoaria in many in- 

 stances present elements common to the whole growth, and 

 not belonging specially to any individual. The chief bond of 

 connection would appear to reside partly in the continuity of 

 the external integument, and partly also, in all probability, in 

 a slow interchange of the vital fluid with which the cavities of 

 the cells are charged." 



In one sub-order of the Polyzoa ( Ctenostomatd), the polyzo- 

 arium consists of a series of cells arising from a common tube, 

 but this exception does not affect the value of the above 

 general distinction between the Polyzoa and the Hydroida. 



A second point of difference is found in the invariably cor- 

 neous (or chitinous) texture of the polypidoms of the Hydroida^ 

 whereas those of the Polyzoa may be corneous or fleshy, but 

 are in the majority of instances more or less highly charged 

 with carbonate of lime. 



As before remarked, the colonies of the Polyzoa are pro- 

 duced by a process of continuous budding from a primitive 

 being or zooid. The budding takes place according to a 

 determinate law, differing in different forms, and the resulting 

 colony varies in shape according to the method of budding in 

 each species. All the zooids of the colony are termed " poly- 

 pides," and the entire colony consists simply of an aggregation 

 of precisely similar polypides, which may be simply united by 

 their external integuments or, more rarely, spring from a com- 

 mon tube. It is only with the outer investment of the colony 

 that the palaeontologist has to deal ; but it may be well briefly 

 to describe the structure of a typical polypide. 



The polypide of a Polyzob'n (fig. 131, 2) consists essentially 

 of a double-walled sac, filled with fluid, and perforated by an 

 aperture where the mouth of the polypide is situated. In the 

 majority of cases the outer wall of the sac (termed the " ecto- 

 cyst") is of a horny consistence, or maybe more or less highly 

 calcareous. It forms a little chamber, which is technically 

 called the " cell." At one point, varying in its position, the 

 cell is furnished with an aperture or "mouth" (fig. 131, T), 

 whence the polypide can protrude its tentaculate head. The 

 inner wall of the sac (termed the " endocyst ") is invariably 

 flexible and membranous, and the space included within it is 

 filled with fluid, in which floats the alimentary canal. The 

 commencement of the alimentary canal is surrounded by a 

 series of hollow ciliated tentacles, which are mostly arranged 



