ANATOMY OF THE SPIDER. 



359 



1', 



Fig. 311. Anatomy of a spider, diagrammatic longitudinal 

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

 to them from the brain (supra-oesophageal ganglion, oG) ; 

 HO, mandibles ; (a?, palpus of maxilla /, ; / 2 , first pair of legs, 

 >i-i s , succeeding pairs ; A", head ; Br, thorax ; H, hind-body 

 or abdomen; Rii, heart or dorsal vessel; i, lung in front of 

 the opening of the oviduct G ; the spinning-glands (up) con- 

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

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



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

 ized by the pos- 

 session of two 

 or three pairs 

 of spinnerets, 

 w h i c h are 

 jointed a p - 

 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 



tracheae, very rudimentary 



mouth-parts, and no brain, 



there being but a single 



ganglion in the abdomen. 

 Order 1. Acarina. 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. 312. /xotfes albipictus from a partly 

 domesticated moose. The tick natural size, 

 gorged with blood, and its six-legged young, 

 much enlarged, a, beak or mandibles armed 

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



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

 irged. 



