MOLLTTSCOIDA : POLYZOA. 



233 



longing specially to any individual. The chief bond of con- 

 nection would appear to reside partly in the continuity of the 



'X 



^ 



Fig. 71. Polyzoa. Flustra hispida, one of the Sea-mats. 1. Portion of the 

 ccenoecium, natural size. 2. A small fragment enlarged to show the 

 cells. 



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 Polyzoa and the Hydroida. 



A second point of difference is found in the invariably 

 corneous (or chitinous) texture of the polypidoms of the 

 Bydroida, 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. 



The homomorphism, however, which subsists between the 

 Polyzoa and the Hydroida is shown most decisively not to be a 

 true aftinity, when the structure of the individual zooids is 



