ARACHNOIDEA. 283 



Class Arachnoidea — Spiders, Scorpions, Mites, etc. 



The class Arachnoidea is far from being a coherent unity. 

 Its subdivisions are numerous and diverse, and the state- 

 ment of general characters is consequently difficult. For 

 the majority, the following hold true. 



The anterior segments, to the number of seven or so, are 

 fused into a cephalothoracic region, which bears six pairs of 

 appendages. Though the most anterior of these appendages 

 is often turned in front of the mouth, there are no pre-oral 

 outgrowths comparable to the antennae of Insects and 

 Myriopods. The first two pairs of appendages (chelicerse 

 and pedipalps) generally have to do with seizing and holding 

 the food, the four other pairs are walking-legs. But although 

 six pairs occur in most, there may be more or less. It is 

 equally unsatisfactory to say that the abdomen is generally 

 but not always without appendages, that it may be segmented 

 or unsegmented, that it is generally distinct from, but may 

 be fused to the cephalothorax. So the respiration may be 

 by tubular tracheae alone, or by chambered tracheee (pul- 

 monary sacs) alone, or by both combined, or (in Limulus) 

 by peculiar gill-like structures. In the tracheate forms {i.e., 

 in almost all except Limulus), there are never more than 

 four pairs of stigmata. An elongated dorsal heart usually 

 lies in the abdomen. 



Systematic Survey of Arachnoidea. 

 Order i. Scorpionidje. Scorpions. 



The Scorpions have a well-developed stinging organ. 

 The venom is lodged in the tip of the " tail," which they 

 bend upwards and wave about when excited. Suddenly 

 straightening it, they strike downwards. In man the poison 

 seems to act chiefly on the red blood-corpuscles, and 

 though never or very rarely fatal the consequences are often 

 serious. " The best remedy is ammonia applied externally, 

 and also administered in small doses internally." 



Scorpions are restricted to warm countries. They lurk 

 under stones, etc., and are active at night. The Scorpio afer 



