ZOOLOGY. 



Order 3. Araneida (Spiders). The Araneida are air-breathing arach- 

 nids, 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 unsegmented 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. 



FIG. 148. 



FIG. 148. Scorpion (Buthus). Photo by Folsom. 



Questions on the figure. Compare the scorpion with figures of Crus- 

 tacea, insects and spiders, noting the chief differences and likenesses. Of 

 what use is the long, segmented abdomen in the scorpion? 



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 cob-web spider, make a complex and irregular mesh-work 

 of fibres running in all directions ; others spin a web similar to the last 

 with the exception that at one point it is continued into a tube into which 

 the spider retreats for hiding. The webs of these spiders are for the 



