THE MYRIAPODS AND ARACHNIDS 77 



the base are two slits which open into the pulmonary sacs 

 which contain the organs of respiration. These organs 

 consist of a series of flattened plates arranged like the 

 leaves of a book and are therefore called the lung books. 

 In addition to these organs spiders are furnished with tra- 

 cheae or air tubes similar to those of insects, but opening 

 near the posterior end of the body. The sex opening 

 occurs just behind the slits leading to the pulmonary sacs. 

 The organs which are the most distinctive of spiders 

 are the spinning organs which produce the material of 



fc's <ye py-g 



FIG. 61. Section through the middle of an orb weaving spider, al.c, 

 alimentary canal opening at a; E, eggs in ovary; I, lung book; cy.g, py.g, 

 and tr.g, cylindrical, pyriform, and tree-like spinning glands respectively; 

 spn. spinnerets. (After McCook.) 



the spider's web. These consist of numerous glands 

 located in the lower part of the abdomen; they open 

 through the spinnerets of which there are usually three 

 pairs near the posterior end of the body. There are in 

 each spinneret a large number of small tubules each of 

 which is connected with a duct from a spinning gland. 

 The material of the web is at first soft and sticky. The 

 spider, after attaching its web by placing its spinnerets 

 against some object, draws out the soft material which rap- 

 idly hardens. As there are numerous spinning tubes the 

 web of the spider, fine as it is, consists of a large number 

 of strands. The web of spiders is used for many pur- 

 poses. It is employed to make the cocoon with which 

 the female surrounds the eggs, in making nests of various 



