126 ARACHNIDA. 



group, consist either of internal air gills performing the office 

 of lungs, and inclosed in pouches, or of radiated trachea, the 

 spiracles or passages for the entrance of the air being from 

 two to eight in number, and situated either at the lower part 

 of the abdomen, or the sides of the cephalo-thorax. 



The anterior part of the cephalo-thorax generally exhibits 

 on its upper surface a certain number of minute shining 

 points, which are the organs of sight ; they vary in number 

 from two to eight, and furnish excellent characters for the 

 distinction of the generic groups. The animals of this class 

 are furnished with eight very long legs, generally terminated 

 by two small claws ; in front of the legs are to be observed 

 a pair of very powerful organs, terminated by acute moveable 

 hooks, which are occasionally of large size (as in Atypus 

 Sulzeri), and, in many cases, afford passages for the discharge 

 of that poisonous fluid with which some of 

 the insects are provided. These organs 

 have been termed chelicera by Latreille, 

 who considers that they represent the an- 

 tennae of insects, and the internal antennae 

 of the decapod Crustacea; but so little 

 attention has been hitherto paid to the 

 comparative anatomy of these analogical or- 

 Atypus Sulzeri. gaus, that it is perhaps the safest course to 

 regard such analogies as undetermined. In 

 addition to such organs, the mouth is furnished with a 

 lower lip and a pair of lateral moveable instruments, similar 

 to the lower jaws of the mandibulated insects, furnished ex- 

 ternally with a pair of jointed appendages or palpi. All the 

 arachnida appear to be carnivorous, but some are parasitic 

 upon the bodies of other animals, and in these the mouth 

 undergoes a considerable change of structure, being com- 

 posed of an instrument capable of suction, although formed 

 of the same typical number of organs as the mouth of the 

 preceding. The portion of the body succeeding the ccphalo- 



