390 



ARTHROPODA 



enlarged into a stercoral pocket, just in front of which the excretory tubules 

 empty. These resemble the Malpighian tubes of insects in function, but 

 differ in being entodermal in origin. Besides there are also coxal glands 

 (modified nephridia), of which only one pair comes to development, 

 and this may lose its external opening on the base of the first or third 



leg. 



The cesophagus is always closely surrounded by a nerve nng composed 

 of brain above and of part of the ventral chain on the sides and below,, 

 the thoracic and more or fewer of the abdominal ganglia entering 

 into its composition (fig. 370, >). Of sense organs, besides tactile hairs, 

 only the eyes (fig. 371), 2-12 in number, are well known. The large 

 number of rods in the retina makes it probable that these eyes see well. 

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

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

 organs, which occur in the skin of body and legs in several groups, is 



unknown. 



The respiratory organs already alluded to (p. 388) have their spiracles, 

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



it is stated, sometimes on the cephalothorax. The 

 internal organs are the lungs and the tracheae. A 

 lung is a rounded sac just inside the spiracle and 

 consists of numerous leaves on the anterior wall 

 of the lung sac. Each leaf contains a blood space 

 in its interior, while between the leaves are flat- 

 tened spaces into which the air enters (fig. 416). 

 The tracheae are branched tubes arising from the 

 abdominal spiracles and penetrating the abdomen 

 (fig. 420). These are lined with chitin, and to 

 strengthen them without undue thickness this 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 tracheae, while in most other arachnids 

 only tracheae occur. (The smaller mites and par- 

 asites lack specialized respiratory organs and circulatory organsas well.) 

 These facts show that lungs and tracheae are morphologically equivalent. 

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

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

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

 have only lungs, the circulation is most nearly complete. 



FIG. 420. Beginning 

 of paired trachea: of 

 Anyphccna accentual a 

 (after Bertkau). st, 

 unpaired spiracle. 



