460 ZOOLOGY. 



these animals, especially in spiders and scorpions, it is con- 

 centrated in pouches lodged in the ahdomen, and called 

 lungs. These latter organs present internally a number of 

 membranous lamellae (I, Fig. 500), arranged like 'the leaves 

 of a book ; thus, they more resemble branchia3 than true 

 lungs. Each lung receives the air by an opening situated in 

 the lower aspect of the abdomen (s), and sometimes two such 

 may be counted, sometimes four, and sometimes eight. 



Certain spiders possess at the same time lungs and tracheae, 

 such as the segestries ;* and others, as the faucheur and 

 the mites, have tracheae only. These tubes have the same 

 structure as in insects, and the air enters them by two very 

 small stigmata situated at the lower part of the abdomen. 



Fig. 499. The Scorpion. 



The blood is white in all animals of this class. The pul- 

 monary arachnida have a circulatory apparatus sufficiently 

 complete. Their heart (Fig. 501), situated in the back, has 

 the form of an elongated vessel, and^ gives origin to various 

 arteries ; the blood, after having traversed the organs, pro- 

 ceeds to the lungs, and from thence reaches the heart, 

 following a course similar to that which we have already 

 seen takes place in the Crustacea ( 111). In the arachnida 

 which breathe only by tracheae, the apparatus of the circula- 

 tion is rudimentary. There appears to be merely a simple 

 dorsal vessel without arteries or veins. 



557. The arachnida lay eggs like insects, and the male 

 differs generally from the female in the form of the maxillary 



* Sub-genus segeatria. Lat. Example of species : Aranea florentina, 

 Koss.-R?K. 



