AUACHNIDA. 



455 



spiders; returning by a less regular channel than the arterial one: 

 the veins of the great cavities of the body are irregular and wide 

 sinuses. 



All true Arachnids breathe the air directly : the tracheary respi- 

 ratory apparatus of the mites commences by a few, usually two, 

 orifices, sometimes concealed between the anterior feet ( Trombi- 

 dium); sometimes very apparent above the third pair of legs ( Ga- 

 tnasus) ; sometimes behind the last pair of legs. In Demodex, the 

 Tardigrada, and P?/ctwgo?iid(s, notraces of respiratory organs have yet 

 been found. In the species, parasitic on the hedgehog (Ixodes Eri?ia- 

 cei), there are three stigmata; two near the sides, and one below, at 

 the middle of the abdomen: the latter is described by Audouin as a 

 pplierical tubercle, pierced by a number of minute. holes, by which 

 the air penetrates the trache£e. They usually arise by a simple 

 tuft from the two stigmata : in Gamasus, there are given off from 

 the tufts two unbranched trachete, whicli course along the sides of the 

 cephalothorax, and terminate in creca at the base of the mouth. 

 The tracheae are very delicate, and their spiral filament discernible 

 only in the larger kinds of mite. Those of the water spiders {Hy- 

 drachtia) doubtless act, like the branchi-trachea) of aquatic insect 

 larva?, in extracting the air from the water. 



The spiders of the genera Segestria and Dysdera have four stig- 

 mata, situated on the under and anterior part of the abdomen : the 

 anterior one on each side (fig- 168, a) 

 is the aperture of the pulmonary sac (6) ; 

 the lower orifice (c) leads to a short and 

 wide cylinder, from which radiate nume- 

 rous trachciB(</) having the usual shining 

 surface. These tubes are united together 

 in bundles, and diverge to the surround- 

 ing parts by dissociation, not by true 

 ramification, like the tracheae of mites 

 and insects. One bundle is dispersed 

 throughout the abdomen ; another enters 

 the cephalothorax and resolves itself 

 into groups corresponding in number 

 with the limbs to the extremity of which 

 the fine silvery trachea can be traced. 

 In Ejyeira and some other spiders there 

 is a transverse fissure in front of the spin- 

 nerets, from which a short tracheal trunk 

 proceeds : this sends off four simple flat- 

 tened tracheae, devoid of the spiral filament, which extend, gradually 



G Ci 4 



168 



Respiratory organs, S gestria. 



