OF THE CLASS ARACHNIDA. 417 



It is also around the anal opening that we find the secre- 

 ting glands of the silky matter and the winders, by the aid 

 of which several arachnida construct webs, which are often of 

 great extent and of extreme fineness (Fig. 414). 



Fig. 413. The Scorpion. 



556. The respiration of the arachnida is ae'rian, and 

 takes place sometimes by means of tracheae ; but in most of 

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

 centrated in pouches lodged in the abdomen, and called 

 lungs. These latter organs present internally a number of 

 membranous lamellae (I, Fig. 414), arranged like the leaves 

 of a book ; thus, they more resemble branchiae 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. 



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

 monary arachnida have a circulating apparatus sufficiently 

 complete. Their heart (Fig. 415), 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 



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

 Ross.-K K. 



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