PTCNOOONIDA. 



339 



^? K oQ 



). — Anatomy of a spider, diaErammatic longitudinal 

 section tlirough the body, au, simple eyes and nerves leading 

 to tliem from the brain (supra-oesophageal ganglion. oCf) ; 

 0712, mandibles ; ta^, palpus of maxilla l^ ; /g, first pair of legs, 

 fi]-^3, succeeding pairs ; K, head ; Br, thorax ; H, hind-body 

 or abdomen; Ric, heart or dorsal vessel ; L, lung in front of 

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

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

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



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

 ized by the pos- 

 H Eii session of two 



Br A^._ir\/ or three pairs 



of spinnerets, 

 which are 

 jointed ap- 

 pendages h o- 

 mologous with 

 the legs. Be- 

 sides tracheae, 

 spiders have a 

 so-called lung 

 (Fig. 306, L), 

 com loosed 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 uiites 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 joairs of long 

 legs, into which coecal prolonga- 

 tions of the stomach pass, as seen 

 in Fig. 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. 



Macroiiotus Americanus Pack, is common in sphagnum 

 swamps. Like the Kotatoria, these low forms are capable 

 of revivifying after being apparently dead and dried up. 



307. — .4mmotftoS pycnogo- 

 stomach with coeca (6, 

 b, b, b) extending into the legs.— 

 From Gegenbaur. 



noides 



