AlfATOMT OF THE SPIDER. 



359 



session of two 

 or three pairs 



spW^ 



Fig. 311. — Anatomy of a epider, diagrammatic longitudinal 

 section through the body, aft, simple eyes and nerves leading 

 to tliem from the brain (supra-cesophageal ganglion, oG) ; 

 a«2' mandibles ; ta^, palpns of maxilla If ; l^, first pair of legs, 

 6,-^n, succeeding pairs ; K, head ; Br, thorax ; H, hind-body 

 or abdomen; _/?«, heart or dorsal vessel ; L, lung in front of 

 the openinf^ of the oviduct G ; the spinning-glands (,s;;) con- 

 nect with the spinnerets, sp W. The digestive tract is shaded, 

 and in the abdomen enveloped in the liver. — After Graber. 



Tlie type of the sub-class is the spider, which is character 



ized by the pos 



H p,u 

 / 



of spinnerets, 

 which are 

 jointed ap- 

 pendages ho- 

 mologous with 

 the legs. Be- 

 sides tracheae, 

 spiders have a 

 so-called lung 

 (Fig. 311, L), 

 composed of 

 several leaves, 

 into which the 

 blood flows, 



and is thus aerated. In Lycosa the blood flows through the 



heart from the head backward. There is a great range of 



structure, from the lowest 



mites to the spiders, certain 



mites having no heart, no 



trachcEe, very rudimentary 



mouth-parts, and no brain, 



there being but a single 



ganglion in the abdomen. 

 Order 1. A carina. — The 



mites are the simplest 



Arachnida, the body being 



oval in form, the head 



usually small, more or less 



merged with the thorax, 



while the latter is not dif- 

 ferentiated from the abdo- 

 men. There is a slight 



metamorphosis, the mite 



when first hatched having 



but three pairs of legs, the fourth (and last) pair being added 



Fig. 3i2.— Ixodes ajbipictvs from a partly 

 domesticated moose. The tick natural size, 

 gorged vfith blood, and its six-logged young, 

 much enlarged, a, beak or mandibles armed 

 with teeth ; 6, maxilla, and c, maxillary pal- 

 pus ; d, a foot with sucker and claws, en- 

 larged. 



