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 structurai features in common. The funda- 
mental distinction between unicellular and multicellular 
animals enables us to separate the Protozoa from the rest, 
which are termed J/efazoa. Hence we have two sub- 
kingdoms, the Protozoa and Metazoa. The Protozoa have 
two phyla, the Gymnomyxa 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 plano-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 #ilateralia, 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 order 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 Sz/ateralia, the first three, 
or the Platyhelminthes, Rotifera, and Nemathelninthes, are of the Archz- 
celic (or Acelomata) type, whereas the other four are Calomata. 
(TABLE. 
