SUPPLEMENT ON ARACHNIDA. 439 



terior feet, or the articulation itself. The eyes are invariably 

 simple, and the feet almost always eight in number. 



The abdomen of the arachuides, like that of the insects, 

 is the seat of the vital functions. The external organs of re- 

 spiration, however, occupy there a more circumscribed space, 

 being exclusively situated on the sides of the belly, or those of 

 the chest ; and not along the whole lateral portions of the 

 body, as in insects. The stigmata lead either to pouches, or 

 sacs enclosing bodies analogous to gills, but performing the 

 office of lungs, or to two trunks of tracheae, which are divided 

 almost from their origin, and in all directions, into a great 

 number of branches. 



The myriapods, the thysanoura, and the parasites, are alone, 

 of all the numerous class of insects, like the arachnida, truly 

 apterous. Like them also, they undergo no metamorphosis, 

 properly speaking, can engender several times, and have their 

 growth not limited to the term of the development of their 

 several organs, or of their aptitude for reproduction. But 

 these insects are, nevertheless, remote from the arachnida in 

 the characters which we have already noticed, or those which 

 are proper to their own peculiar class. 



Among the arachnida, some have two articulated man- 

 dibles, terminating in a talon or pincers, similar to small feet ; 

 two palpi still more analogous to locomotive organs ; two, or 

 several jaws, formed by the dilatation of those palpi, or of the 

 anterior pair of feet ; and a lip without palpi. The other 

 arachnida have a mouth after the manner of a sucker, but the 

 pieces of which, though otherwise modified, appear to cor- 

 respond with the preceding. It is also, most frequently, 

 accompanied with two palpi. The number of simple eyes 

 varies from two to eight. Their situation, their symmetrical 

 disposition, their relative sizes, and their forms, often furnish 

 to the naturalist the appropriate means of distinguishing the 

 principal divisions. 



