GHAPTEE XIII 



ARTHROPODA (continued) 



Class II. : ARACHNIDA 

 (Spidees, Harvestmbn, and Mites) 



The class Arachnida is a very large one, including 

 Characters ^^^Y diverse orders, of which only three will be 



mentioned here, viz. the Araneae or true Spiders, 

 the Phalangidea or Harvestmen, and the Acarina or Mites. 



All the Arachnida are alike in being air-breathing forms, 

 and in having the head and thorax fused together to form 

 the "cephalo- thorax." The head bears no true antennae, 

 and only two pairs of jaws. There are four pairs of walking 

 legs. The abdomen is segmented in some and not in others ; 

 in the Mites it is not differentiated from the thorax. The 

 sense organs are simpler than in other Arthropods, the eyes 

 being simple and sessile ; there is as a rule no metamorphosis 

 in the development, the young being like the adult except 

 for size. 



Order I. : Aranbae (The True Spiders) 



Spiders are Arachnids in which the abdomen is unseg- 

 mented, and separated from the cephalo-thorax by a narrow 

 constriction or "waist." On the under side of the abdomen, 

 rather near its apex, are special spinning organs or spinnerets, 

 from which are produced the silk threads with which a spider 

 spins her characteristic snare or web. The head bears no 

 antennae. It has usually eight simple eyes, but the number 

 varies in different species ; the mouth is flanked by two pairs 

 of appendages — a pair of two-jointed jaws, chelicerae (often 



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