CLASSIFICATION 39 



reason to believe that in certain cases it orig- 

 inates by a folding off of part of an original 

 cavity corresponding with that of Hydra ; so that 

 part went to form the intestine, and part the 

 cavity surrounding it. 



The above arrangement of the main great 

 groups of animals into four grades is that given 

 by Professor Arnold Lang. 



It should be added, that there are a few ex- 

 ceptional forms that present a departure from 

 these broad rules of structure. They are, how- 

 ever, so few that they need only be named as cu- 

 riosities. For instance, there are parasites in 

 which the inner body-layer is practically done 

 away with, because they are fitted to absorb food 

 through the outer layer. And in one division of 

 the Moss-Corals there is no body-cavity to be 

 seen, although it is to be found in the other di- 

 vision. 



What is the outcome of all this sorting of the 

 animal kingdom? This most important result: 

 that a classification of the animal kingdom into 

 the four grades we have named, presents, in serial 

 order, the stages through which young animals of 

 the higher forms pass in the course of their 

 growth. Every creature begins as a unicellular 

 organism the fertilised egg-cell. A vast num- 

 ber of creatures belonging to the higher groups 

 present, later on, a two-layered condition, com- 

 parable with that of Grade II. Later on they 

 acquire a third layer, and therefore correspond 

 with Grade III. By degrees the body-cavity is 

 formed, and they then present the adult body- 

 structure of Grade IV. The development of the 

 chicken in the egg, for instance, presents these 

 four stages. 



