PYCNOGONIDA. 



339 



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

 ized by the pos- 



H _Rii session of two 



or three pairs 

 of spinnerets, 

 which are 

 jointed a p - 

 pendages ho- 

 mologous with 

 the legs. Be- 

 sides tracheae, 

 spiders have a 

 so-called 

 306, 



la. 



lung 



Fig. 366. Anatomy of a spider, diagrammatic longitudinal 

 section through the body. ?/, simple eyes and nerves leading 

 to them from the brain (supra-eesophageal ganglion, oCf) ; 

 2 , mandibles ; ta, palpus of maxilla I, ; /,, first pair of legs, p n in r> n <? P f\ of 

 6,~* 3 , succeeding pairs ; A', head ; Br. thorax ; H, hind-body 



or abdomen; ft a, heart or dorsal vessel; L, lung in front of several leaves, 

 the opening of the oviduct G ; the spinning-glands (.9/7) con- . . 



nect with the spinnerets, sp W. The digestive tract is chaded, into Which the 

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



Fig. 307. Ammothoi? pycnogo- 

 noides. a, stomach with coaca (6, 

 6, 6, 6) extending into the legs. 

 From Gegenbaur. 



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. 



f The Pycnogonida, Tardigrada, and Linguatulina are probably 

 independent classes of Arthropods. 



