POLYZOA OR BRYOZOA. 415 



external integument ; whereas the zooids of the latter are 

 united by an organic connecting medium, or " ccenosarc," 

 from which they take their origin. On this point Mr Busk 

 observes : 



" It has been before said that the Polyzoa are always asso- 

 ciated into compound growths, made up of a congeries of 

 individuals, which, though distinct, yet retain some degree 

 of intercommunication, comparable in kind perhaps, though 

 not in degree, to what obtains in many of the compound 

 Ascidians. That this community exists is proved by the 

 otherwise inexplicable circumstance that the polyzoaria in 

 many instances 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 con- 

 tinuity 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 (Ctenostomata), the poly- 

 zoarium 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 Pofyzoa and the Hydroida. 



A second point of difference is found in the invariably 

 corneous (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 

 " polypides," 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 common tube. It is only with the outer invest- 

 ment of the colony that the palaeontologist has to deal ; but 

 it may be well briefly to describe the structure of a typical 

 polypide. 



