ARTHROPODA 49 



all parts of the body. We can, therefore, make two 

 divisions of the Phylum Arthropoda: 



Division I. Branchiata breathing in water, through the general 



surface of the body or through gills. 

 Division II. Tracheata breathing air, through tracheae or in some 



other way. 



The first division contains only the class Crustacea, and 

 the Tracheata therefore include the other four classes. 

 Two of the latter have retained the ancestral worm-like 

 form, with metameric appendages, but are easily dif- 

 ferentiated from each other because one class (Myriapoda) 

 has lost the primitive metameric kidneys completely and 

 the other has retained them (Onycophora). The other 

 two classes have departed from the primitive worm-like 

 form and are highly specialized. They have lost the 

 appendages on the posterior end of the body; lost the 

 metameric kidneys and developed a new type of excretory 

 organ (the Malpighian tubules); and become specialized 

 in many other ways. One of these last two classes (Insecta) 

 has kept only six walking legs and has developed wings, 

 the other (Arachnida) has retained eight walking legs and 

 never possesses wings. We may at present, therefore, 

 classify the Phylum Arthropoda on the following basis: 



Division I. Branchiata. Aquatic, breathe through skin or gills. 



Class 1. Crustacea. Aquatic arthropods, with primitive but 



nori-metameric kidneys. 



Division II. Tracheata. Terrestrial (or aquatic), breathe air, usually 

 through tracheae. 



Class 2. Onycophora. Worm-ljke, with paired metameric kid- 

 neys throughout the body. 



Class 3. Myriapoda. Worm-like; excretion through Malpig- 

 hian tubules which .open into the intestine just behind the 

 stomach. 



Class 4. Insecta. Three pairs of walking legs; one or two pairs 

 of wings usually present; no metameric kidneys, Malpighian 

 tubules present. 



Class 5. Arachnida. Four pairs of walking legs; no wings; 

 Malpighian tubules present. 



