ANNULOSA ARACHNIDA. 



the atmosphere by stigmata. The walls of these bags, 

 which are termed pulmonary sacs, are abundantly 

 supplied with blood- 

 vessels. Sometimes 

 there are both tracheae 

 and pulmonary sacs. 

 In the lowest groups 

 there are no special 

 breathing organs, the 

 blood being aerated 

 by the general sur- 

 face of the body. 



87. The Nervous 

 System consists of 

 the usual chain of 

 ganglia, but is more 

 condensed than in the 

 Insecta. There is a 

 cephalic ganglion 

 above the 03sophagus, 

 and a large ganglion 

 placed in the thorax. 

 There is sometimes 

 also a small ganglion 

 in the abdomen. 

 These ganglia are 

 united by a pair of 

 filaments, and give 

 off nerves to the 

 various organs of the 

 body. 



88. The eyes, 

 which are always 



simple, are placed in the front of the cephalothorax. 

 They vary in number from two to eight. 



89. Development. The Arachnida are generally ovi- 

 parous, and, with some exceptions, unisexual. In most 

 cases, the young animal resembles its parent in every- 



FIG. 28. ANATOMY OF SCORPION*. 

 a, the dorsal vessel and arteries ; &, alimen- 

 tary canal ; b' , anus ; c, chain of nervous ganglia ; 

 e' , one of the eyes and its optic nerve ; dd, pul- 

 monary sacs; c, terminal hooked claw with 

 poison gland. 



