300 



ARTHROrODA 



enlarged into a slcravjl pofkti, just in front of wliich the excretory tubules 

 empty. These resemlile tire Malpigliian tubes of insects in function, but 

 differ in being entodermal in origin. Besides tliere are also coxal glanils 

 (modified nephriiha), of \vluch only one pair comes to development, 

 and tliis may lose its external opening on the base of fJie lirst or third 

 leg. 



The cxsophagus is always closely surrounded by a nerve ring composed 

 of brain abo\'e and of part of tire ventral chain on the sides and below, 

 the thoracic and more or fewer of the abdominal ganglia entering 

 into its composition (tig. 370, D). Of sense organs, besides tactile hairs, 

 only tire eyes (fig. 37r), 2-12 in numl.ier, are well known. The large 

 number of rods in tire retina makes it probaftle that these eyes see well. 

 Hearing is well developed, but it is uncertain whether certain hairs oir the 

 legs and palpi are the auditory organs. The function of the lyriform 

 organs, wliich occur in the sldn of body and legs in several groups, is 

 unknown. 



The respiratory organs already alluded to (p. 3SS) have their spiracles, 

 always few in itumber, on the anterior ventral part of the abdomen and, 

 it is stated, sometimes on the cephalotliorax. The 

 internal organs are the lungs and the trachea-. A 

 lung is a rounded sac just inside tlie spiracle and 

 consists of numerous lea\"es on the anterior wall 

 of the lung sac. Each leaf contains a blood space 

 in its interior, wliile between the leaves are tlat- 

 tened spaces into wliich the air enters (fig. 41(1). 

 The trache;r are branched tubes arising front the 

 abdominal spiracles and penetrating the abdomen 

 (fig. 4-o)- These are Uned with cliitin, and to 

 Ty^'-y ^^S^ strengthen them without undue tliickness tliis lining 

 is thrown into folds, usually arranged in a spiral. 

 In the scorpions and tetrapneumonous Araneina 

 only lungs occur. In other spiders one pair of lungs 

 is replaced by trachea-, wliile in most other arachnids 

 only trachea- occur. (The smaller mites and par- 

 asites lack specialized respiratory organs and circulatorv organsas well.) 

 These facts show that lungs and trachea- are morphologically equi\'alent. 

 The localization of respiration in the abdomen has resulted in having the 

 heart in the same region. It is noticeable that, as the trachea- are de- 

 veloped, the circulatory vessels are reduced. In the scorpions, which 

 have only lungs, the circulation is most nearly complete. 



Frc. 420. — Bcs^innins; 

 of paired trachea' of 

 Anyplucna aici-iiluala 

 (after Bcrlkau). si, 

 unjiaireJ spiracle. 



