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 arachnides, 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 w T e 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 tw r o 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. 
