TRACHEA UliE. 207 



two to four(l). The want of sufficiently general anatomical 

 observations, has prevented the limits of this order from 

 being rigorously determined. Some of these Arachnides, the 

 Pycnogonides for instance, exhibit no stigmata; their mode of 

 respiration is unknown. 



The Trachearise are very naturally divided into those which 

 are furnished with chelicera?, terminated by two fingers, 

 one of which is movable, or by one that is equally so; and 

 into those where these organs are replaced by simple laminae, 

 or lancets, which with the ligula constitute a sucker. Most 

 of these animals, however, being very small, great difficulties 

 necessarily accompany these investigations, and it is readily 

 perceived that such characters should only be resorted to 

 when it is impossible to avoid it. 



FAMILY I. 



PSEUDO-SCORPIONES. 



In this family we find the thorax articulated, its first seg- 

 ment much the largest, and resembling a corselet ; the abdomen 

 is very distinct and annulated, and the palpi very large and in 

 the form of legs or claws. There are eight legs in each sex, 

 with two equal hooks at the extremity of the tarsi, the two 

 anterior ones, at most, excepted, and two apparent chelicerse 

 terminated by two fingers and two toes, formed by the first 

 joint of the palpi. They are all terrestrial, and have an oval 

 or oblong body. This family comprehends but two genera. 



the distribution of the blood to the different parts of the body, and its return from 

 the organs of respiration to the heart. Thus, although some vessels have recently 

 been discovered in certain Insects Phasma: and, although they may possibly 

 exist in various Arachnides Trachearise, it does not exclude them from the general 

 system. M. M. de Serres has observed that the intestinal tube of the Phalangium 

 gives off numerous caeca or vermiform appendages, which seem to have some 

 analogy with the hepatic vessels, and that the tracheae ramify over them ad infi- 

 nitum. 



(1) According to Mullcr the Hydrachuu umbrata has six eyes: but may this 

 not have arisen from an optical illusion or some mistake? 



