THE CLASSIFICATION OF ANIMALS 45 



of other parts, and in the adult is represented by small sac-like 

 cavities, from the walls of which the reproductive cells arise. 



The Arthropoda are an enormous phylum, and include more 

 than half a million kinds of animal, of which accurate descrip- 

 tions have been published. They affect and limit the life of 



FIG. 17. A water- flea as an example of Crustacea 

 viewed from below ; at 1 , first antennae ; at*, 

 second antennae ; gn, gnathites ; I, walking 

 legs ; o, mouth. 



man more than any other group of the animal kingdom. 

 Leaving out of account minor divisions, the Arthropoda are 

 subdivided into three main groups. We have first of all 

 the Crustacea, which are almost all aquatic, and which have 

 the first two pairs of limbs, which are situated in front 

 of the mouth, transformed into delicate feelers or antennae, 

 and the following two pairs, and often others, transformed into 



