PYCNOGON1DA. 



339 



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

 ized by the pos- 

 session of two 

 or three pairs 

 of spinnerets, 

 which arc 

 jointed a p - 



ho- 

 with 



Be- 



la 



pendages 

 mologous 



the legs, 

 sides tracheae, 

 spiders have a 



Fig. 366. Anatomy of a spider, diagrammatic longitudinal S0 



section through the body, aw, simple eyes and nerves leading (fjo- 306 L) 

 to them from the brain (supra-cesophageal ganglion. oG) ; 



an y . mandibles ; ta 2 , palpus of maxi'la I, ; U, first pair of legs, c O m p O S C d of 

 6,-i 3 , succeeding pairs ; A', head ; Br. thorax ; //, hind-body L 



or abdomen; Jfu, heart .or dorsal vesi-el ; L, lung in front of SOVenil leaves, 



the opening of the oviduct G ; the spinning-glands (*p) con- . , . , , , 



nect with the spinnerets, sp II'. The digestive tract is c haded, HltO WlllC'll tllC 



and in the abdomen enveloped in the liver. After Graber. , -, ] fl -o 



and is thus aerated. Iri 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 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. 



Macrobiofus Americanus Pack, is common in sphagnum 

 swamps. Like the Rotatoria, these low forms are capable 

 of revivifying after being apparently dead and dried up. 



J 



Fig. 307. AmmotJioe pycnogo- 

 noides a, stomach with coaca (6, 

 b, b, b) extending into the legs. 

 From Gegenbaur. 



