122 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



the tip being perforated by a myriad of little tubes, 

 through which the silk escapes in excessively fine threads. 

 An ordinary thread, just visible to the naked eye, is the 

 union of a thousand or more of these delicate streams of 

 a fluid which, like collodion, hardens on exposure to the 

 air. 23 



The mandibles are vertical, and end in a powerful 

 hook, in the end of which opens a duct from a poison 



FIG. 81. A, female Spider; B, male of same species; C, arrangement of the eyes. 



gland in the head (Fig. 216). The maxillae, or " palpi," 

 which in scorpions are changed to formidable claws, in 

 spiders resemble the thoracic feet, and are often mis- 

 taken for a fifth pair. The brain is of larger size, and 

 the whole nervous system more concentrated than in the 

 preceding order. There are generally eight simple 

 eyes, rarely six. They breathe both by tracheae and 



