CHAPTER XIX 



POLYZOA (continued) 



CLASSIFICATION GEOGRAPHICAL DISTRIBUTION PALAEONTOLOGY 



METHODS FOR THE EXAMINATION OF SPECIFIC CHAR- 

 ACTERS TERMINOLOGY KEY FOR THE DETERMINATION OF 



THE GENERA OF BRITISH MARINE POLYZOA 



Our account of the Polyzoa would be manifestly incomplete 

 without some reference to the systematic arrangement of these 

 animals. An outline of the principal groups has been given on 

 p. 475. So far, the classification is easy, but it is otherwise when 

 we attempt to subdivide most of the groups any further. 



Systems of classification which depend exclusively upon the 

 external characters of animals have been repeatedly shown to be 

 unsatisfactory. Now with regard to the Polyzoa, not only is it 

 the case that the great majority of forms are only known in their 

 external characteristics, but current systems of classification 

 cannot be regarded as final, because it is not yet certain which of 

 the external features have most systematic value. Two obvious 

 points can be at once selected namely, the character of the 

 zooecium and the character of the entire colony. One or two 

 instances will serve to show what different results are obtained 

 by depending exclusively on either of these characters by itself. 



According to the older writers, the habit of the colony was 

 taken as the most important generic character ; and there can 

 indeed be no doubt that this feature has great importance within 

 certain limits. Any one who has examined different species of 

 such genera as Flustra, Cellaria, Bugula, Metepora, etc., must feel 

 that the form of the colony goes for a good deal. But a con- 

 sideration of other cases shows that there is great risk in the 



