CLASS ARACHNID A. 35 



two pairs of antennae, and that there is a distinct heart 

 placed upon the back. By these characters the class of 

 the Cmstacea is distinguished as a whole. 



CHAPTER X. 



CLASS AEACHNIDA. 



The class Arachnida (Greek, arachie, a spider) comprises 

 the Scorpions, Spiders, Mites, and Ticks. As an excellent 

 example of this class we may select the common House 

 Spider (Tegenaria civilis) of Britain. The body of this 

 familiar animal (fig. 15, A) in reality resembles that of the 

 Lobster, in being composed of a series of rings or segments, 

 placed one behind the other ; but these segments are not 

 conspicuous, and the body only shows a well-marked divi- 

 sion into two distinct portions — a front portion carrying 

 the legs, and a hinder portion carrying no appendages. 



The skin over the whole body is more or less hardened 

 with horny matter; but more so in some parts than in others, 

 and it nowhere forms a shell like that of the Lobster. 



The front portion or haK of the body (fig. 15, A, c) is in 

 reality composed of the head and trunk ("thorax"), so 

 consolidated together that no sign of a boundary between 

 them can be made out, and that no distinct segments can 

 be detected. On the sides of this region of the body we 

 observe four pairs of long, jointed legs, and in front of 

 these a pair of what look like small legs. These latter 

 organs (p), however, are not really legs, but are a sort of 

 feelers which are attached to the jaws. The spider, there- 

 fore, has truly four pairs of legs, and it should thus never 

 be confounded with the genuine Insects, all of which pos- 

 sess no more than three pairs of legs. All the legs are 

 long and slender, covered with nimierous short hairs and 

 a few longer spines, terminated by three claws each, and 



