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 antenne and mandibles of other 
Arthropods. In the scorpions and some species of mites 
these are furnished 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. 
Respiration is affected by means of trachee, 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 
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