28 CLASSIFICATION. 



recognising a natural affinity (for it is a relative term) and there is no 

 question that our classifications are still very unnatural. 



All that can with present knowledge be done in classifi- 

 cation of the animal kingdom is to distinguish certain 

 large Phyla or branches, the members of which have certain 

 important structural features in common. The funda- 

 mental distinction between unicellular and multicellular 

 animals enables us to separate the Protozoa from the rest, 

 which are termed Metazoa. Hence we have two sub- 

 kingdoms, the Protozoa and Metazoa. The Protozoa have 

 two phyla, the Gym^iomyxa and Corticata, and the Metazoa 

 several important phyla. The two lowest of these differ 

 from the rest by being typically axo-symmetric, retaining 

 the primary axis of the gastrula, whilst the rest are 

 primitively piano-symmetric about a plane at right angles 

 to the primary axis of symmetry. This important dis- 

 tinction is emphasised by the two divisions of Protaxonia 

 and Bilateralia^ the latter being all tridermic. 



The Phyla are divided into sub-phyla and classes, the 

 characters of which depend mainly upon general com- 

 munity of structural design. Finally, the classes are 

 further sub-divided into orders, families, and genera until 

 the species is reached. 



The various groups are not in all cases exactly compar- 

 able, but the same oider is always pursued in dividing 

 up a phylum. 



The list here given includes all the more important 

 phyla which are dealt with in this work and their division 

 into classes. 



It will be seen that, of the phyla of the Bilateralia, the first three, 

 or the Platyhelminthes, Rotifei-a, and Nemathelminthes, are of the Airhi- 

 coelic for Accelomata) type, whereas the other four are Ccelomata. 



[table. 



