258 MANUAL OF ZOOLOGY SECT. 



or external openings of which have already been referred to. 

 Each pulmonary sac is a compressed chamber lined with a 

 thin cuticle. The lining membrane is raised up into numer- 

 ous delicate laminae lying parallel with one another like the 

 leaves of a book. Into the numerous narrow spaces be- 

 tween the laminae the air penetrates, and oxygenates the 

 blood, which enters the interior of the laminae from the 

 ventral blood-sinus. 



The Spiders (Fig. 143) differ from the Scorpions in 

 having the abdomen short, rounded, and unsegmented, 



FIG. 143. Spider (Epeira diadema). 



while the chelicerae are subchelate and provided with poison- 

 glands the ducts of which open at their extremities, and 

 the pedipalpi are simple, the terminal joint in the male being 

 expanded, and the whole appendage being used as an intro- 

 mittent organ for the transference of the sperms to the 

 genital opening of the female. At the extremity of the 

 abdomen is a peculiar apparatus, the arachnidium or spin- 

 ning organ. This consists of four or six elevations, the 

 spinnerets, on the surfaces of which open the numerous ducts 

 of the spinning glands : these secrete the material of which 



