CHAPTER II: THE EXTERNAL ANATOMY OF 



SPIDERS 



Spiders resemble the allied animals described in the preceding 

 pages in having the segments of the body grouped in two regions, 

 the cephalothorax (ceph-a-lo-tho'rax) and the abdomen (Fig. 71), 

 in having four pairs of legs fitted for walking, and in having the 

 antenna? modified into organs of prehension, the chelicera:. 



The spiders differ from other Arachnida in having the ab- 

 domen unsegmented and joined to the cephalothorax by a narrow 

 stalk. There is a single small family of spiders, Liphistiidtv, in 

 which the abdomen is segmented; but representatives of this 

 family have been found only in the East Indies. In the genus 

 Tetrablemma, found in Ceylon, the abdomen bears a series of 

 plates, which are evidently vestiges of a segmented condition. 

 But in all American spiders the abdomen is sac-like. 



THE CEPHALOTHORAX 



In spiders the cephalothorax, like the abdomen, is unseg- 

 mented; although frequently the head and the thorax are slightly 

 separated by a furrow, the cervical groove (Fig. 72). In such cases, 

 most writers refer to the head as the cephalic part, or the pars 

 cephalica, and to the thorax as the thoracic part, or the pars 

 thoracica; but the simple terms head and thorax are sufficient 

 for all purposes, and will be used in this book when it is necessary 

 to refer to the principal divisions of the cephalothorax. The 

 hard integument forming the dorsal wall of the cephalothorax 

 is termed the carapace (car'a-pace). 



THE HEAD 



The head is that portion of the cephalothorax which bears 

 the eyes and the so-called mouth-parts, the appendages that are 

 used for seizing and chewing their prey. It is either slightly 

 or not at all separated from the thorax; but it is almost always 



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