ARTHROPODA 



307 



tacea, with which it was formerly classified. Numerous related forms flourished 

 earlier in the world's history but are now extinct (Trilobites). 



Order II. Scorpionida (scorpions). Arachnids with a much elongated and 

 segmented abdominal region closely connected with the thorax. They are air- 

 breathers, with four pairs of book-lungs in the abdomen. The posterior abdominal 

 segments form a tail the last segment of which bears a sting. See Fig. 150. 



FIG. 150. 



FIG. 150. Scorpion (Buthus). Photo by Folsom. 



Questions on the figure. Compare the scorpion with figures of Crustacea, 

 insects and spiders, noting the chief differences and likenesses. Of what use is 

 the long, segmented abdomen in the scorpion? 



Order III. Araneida (spiders). The Araneida are air-breathing arachnids, 

 with book-lungs alone or in connection with tracheae. Poison glands are common 

 in connection with the first pair of (mouth) appendages. The abdomen is un- 

 segmented and without appendages, unless the spinnerets represent reduced 

 appendages. On these latter, open the ducts of the numerous glands secreting 

 the fluid which hardens on exposure to the atmosphere and makes the silk of the 

 web. 



Spiders may be classified on the basis of the type of web which they make. 

 The "orb-weavers" construct webs of great regularity and beauty; others, as the 



