The Internal Anatomy of Spiders 



The blood passes from the body-cavity into the lumen of each 

 leaf, and the respiratory process takes place through the walls 

 f the leaves. 



A hint regarding the mode of origin of this highly specialized 

 respiratory apparatus is afforded by the comparatively simple 

 lung-sacs of the Microthelyphonida, referred to on page 13. 



The tubular trachea. — The distribution of tubular tracheae 

 among the different families of spiders, and the number of tracheal 

 spiracles are indicated by Fig. 152. In most cases the tracheae 

 open by a single spiracle, situated on the middle line of the ventral 

 aspect of the abdomen; sometimes this spiracle is near the middle 



k. 



Fig. 156. 



TUBULAR TRACHE/E 



OF ANYPH.ENA 



(after Bertkau) 



Fig. IS7- 



SECTION OF HEART 



AND PARTS LYING ABOVE IT 



(after Vogt et Yung) 



of the length of the abdomen, but usually it is only a short dis- 

 tance in front of the spinnerets. In a few spiders there is a pair 

 of tracheal spiracles situated just behind the lung-slits. These 

 can be distinguished from lung-slits by the absence of external 

 indications of book-lungs. 



The tracheae are paired organs even when they open by a 

 single spiracle; this is well-shown by the figure of the main trunks 

 of the tracheae of Anyphana (Fig. 156). As to the histological 

 structure of the tracheae, they, like the tracheae of insects described 

 in many text-books, present the same layers as does the body-wall, 

 namely, a chitinous intima lining the cavity and continuous 

 with the cuticula of the body-wall; an epithelial layer surrounding 



147 



