TYPE ARACHNID A. 435 



CHAPTEE XIV. 



TYPE AKACHNIDA. 



The Arachnida are essentially terrestrial forms, for thougli 

 a few species lead an aquatic or marine life, they are evi- 

 dently descendants of forms which led a terrestrial existence 

 and have only secondarily acquired the power of living under 

 water. In all members of the group the body is covered by 

 a more or less thick chitinous cuticle and the appendages are 

 as a rule jointed. 



A characteristic feature of the group is the fusion of the 

 head and thorax to form an unsegmented cephalothorax bear- 

 ing usually six pairs of limbs. The first pair of these are 

 the vhelicerce (Fig. 201, cJi), composed of one to three joints 

 and terminated either by a claw or a chela ; they lie in front 

 of the mouth, which is bounded at the sides by the basal 

 joints of the second pair of appendages, the pedipalps (pe), 

 which may be long and limblike, or chelate, or in some cases 

 clawlike, their basal joints serving in all cases as mandibles. 

 Behind these follow four pairs of legs composed of six or seven 

 joints, the basal joint being termed the coxa, the next, usually 

 short, the trochanter, the third the femur, the next two to- 

 gether form the tibia, then follows in some forms a metatarsus, 

 while the terminal one, provided with two claws, termed 

 ungues, and in some mites also with a suctorial disk, consti- 

 tutes the tarsus. Variations from this structure of course 

 occur, the chelicerse, for example, in some mites being re- 

 duced to short stylets, and in others the two posterior pairs of 

 legs may be quite rudimentary {Phytoptus). The most impor- 

 tant variation is, however, that found in the members of the 

 order Solifugje, in which a head is distinctly marked off from 

 a thorax composed of three segments. 



The abdomen in some forms is segmented, in others all 

 trace of the segmentation is lost, and, finally, in the Mites it 



