290 COMPARATIVE ZOOLOGY. 



union of a thousand or more of these delicate streams of 



silk. 160 These primary threads are drawn out and united 



by the hind legs. 



The mandibles are vertical, and end in a powerful hook, 



in the end of which opens a duct from a poison-gland in 

 the head. The maxillae, or " palpi," which 

 in Scorpions are changed to formidable claws, 

 in Spiders resemble the thoracic feet, and are 

 often mistaken for a fifth pair. The brain is 

 of larger size, and the whole nervous system 



FIG. 26i. spin- more concentrated than in the preceding or- 

 der, b,c-, a! pal- der. There are generally eight simple eyes, 



piform organs. rarely gjx> They breathe both by trac j iege 



and lung-like sacs, from two to four in number, situated 

 under the abdomen. All the species are carnivorous. 



The instincts of Spiders are of a high order. They are, 

 perhaps, the most wily of Articulates. They display re- 

 markable skill and industry in the construction of their 

 webs; and some species (called "Mason Spiders") even 

 excavate a subterranean pit, line it with their silken tapes- 

 try, and close the entrance with a lid which moves upon a 

 hinge." 1 



CLASS III. Myriapoda. 



Myriapods differ from Crustaceans and Spiders in hav- 

 ing the thorax merged in the abdomen, while the head is 

 free. In other words, the body is divided .into similar 

 segments, so that thorax and abdomen are scarcely distin- 

 guishable. They resemble Worms in form and in the 

 simplicity of their nervous and circulatory systems ; but 

 the skin is stiffened with chitine, and the legs (indefinite 

 in number) are articulated. The legs resemble those of 

 Insects, and the head appendages follow each other in the 

 same order as in Insects eyes, antennae, mandibles, max- 

 illae, and labium. They breathe by tracheae, and have two 

 antennae and a variable number of eyes. 



