PYONOOOmUA. 



339 



an.,; 



lt3l,-„- 



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

 ized by the pos- 

 H Rii session of two 



or tliree pairs 

 of spinnerets, 

 which are 

 jointed ap- 

 pendages ho- 

 mologous with 

 the legs. Be- 

 sides tracheae, 

 ep st)Wf> spiders have a 



Fig. 366.— Anatomy of a spider, diagrammatic longitudinal SO-Called lung 



section tlirougli the body, aw, simple eyes and nerves leading CTfirr ^Ofi T^ 



to them from the brain (supra-CEsophageal ganglion, off) ; \ &• """5 ■'-';> 



ore,, mandibles ; ta^, palpus of maxilla ;, ; U, first pair of legs, f>nTnnnsprl nf 



6,-i,, succeeding pairs ; A" head ; Sr, thorax ; // hind-body >- " "^ F " » ^^ ^^ "'■ 



or abdomen; Ha, heart or dorsal vessel ; L, lung in front of several IcaveS 



the opening of the oviduct ff ; the spinning-glands (»p) con- . , _ . _ , _ 



nect with the spinnerets. »p W'. The digestive tract is ^ haded, mto Whlch the 



and m the abdomen enveloped in the liver.— After Graber. , , -, n 



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. — The Pycnogonida 

 are marine forms, without air- 

 tubes, with four pairs of long 

 legs, into which co3cal prolonga- 

 tions of the stomach pass, as seen 

 in Pig. 307. 



Order 2. Tardigrada. — The 

 bear animalcules (Fig. 308) are 

 related to the mites. In these 

 singular beings the ovary and 

 testis exist in the same individual. 



Macrobiotus Americanus Pack, is common in sphagnum 

 swamps. Like the Rotatoria, these low forms are capable 

 of revivifying after being apparently dead and dried up. 



Fig. 307. — AmmotTwS pycnogo- 

 noides. a, stomach with cceca (b, 

 6, b, b) extending into the legs.— 

 From Gegenbaur. 



