44 DEFINITION OF THE TERM INSECT. 
skeleton of the animals just named; and that even in those 
in which the body appears to have no such segments, as 
in many of the Mites (Acarina), they are to be found in 
the limbs. This last circumstance, to have externally 
jointed legs, is the peculiar and most general distinction by 
which the Jnsecta of Linné, including the Crustacea, may 
always be known from the other invertebrate animals*. 
If we proceed further to examine the substance of this 
crust or covering, though varying in hardness, we shall 
find it in most cases, if we exclude from our considera- 
tion the shells of the Mollusca, &c. better calculated to 
resist pressure than that of the majority of animals that 
have no spine. In all the invertebrate tribes, indeed, the 
muscles, there being no internal skeleton, are attached 
to this skin or its processes, which of course is firmer 
than the internal substance; but in insects it is very often 
rigid and horny, and partially difficult to perforate, sel- 
dom exhibiting that softness and flexibility which is found 
in the cuticle of birds and most quadrupeds. From this 
conformation it has been sometimes said, that insects 
carry their bones on the outside of their body, or have 
an external skeleton. ‘This idea, though not correct in 
all respects, is strictly so in this—that it affords a general 
point of support to the muscles, and the whole structure 
is erected upon it, or rather I should say within it. The 
difference here between Jnsects and the Vertebrata seems 
very wide; but some of the latter make an approach to- 
wardsit. I allude to the Chelonian Reptiles ( Zestudo L.), 
® The Annelida have, however, sometimes jointed organs, which 
facilitate their progressive motion whether vermicular or undulatory ; 
but they cannot be deemed /egs, since they neither support the body 
nor enable it to walk, &c. Latreille Anim. invertebr. Artic. 126. Ann. 
du Mus, 1821. 
‘ 
