CHAPTER XI 



ARTHROPODS (Continued). CLASS ARACHNIDA 



128. General characters. In this group, comprising the 

 spiders, mites, and a large assemblage of related species, we 

 again meet with great differences in form and structure 

 which fit them for lives under widely different conditions. 

 The three regions of the body, head, thorax, and abdomen, 

 so clearly marked in the insects, are here less plainly de- 

 fined. The head and thorax are usually closely united, and 

 in the mites the boundaries of the abdomen are also indis- 

 tinct. The appendages of the head are two in number, and 

 probably correspond to the antenna? and mandibles of other 

 Arthropods. In the scorpions and some species of mites 

 these are furnishw with pincers for holding the prey, and 

 in other forms they act as piercing organs. Usually the 

 thorax bears four pairs of legs, a characteristic which readily 

 separates such animals from the insects. 



The internal organization differs almost as much as does 

 the external. In many species it shows a considerable re- 

 semblance to that of some insects, but in others, especially 

 those of parasitic habits, it departs widely from such a type. 

 Eespiration is affected by means of tracheae, or lung-books, 

 which consist of sacs containing many blood-filled, leaf -like 

 plates placed together like the leaves of a book. 



Usually, as in the insects, the young hatch from eggs 

 which are laid, but in the scorpions and some of the mites 

 the young develop within the body and at birth resemble 

 the parent. Almost all of these organisms live either as 



133 



