Invertebrate Animals. f 283 
It thus appears, from what has been said, that between the 
two chief divisions of invertebrate animals which with Lin- 
neeus we term Insecta and Vermes, there is a clear distinction, 
nay, in many instances, a marked contrast, inasmuch as in- 
sects are distinguished from worms by possessing a system of 
peculiar organs of, motion, by the division of the mass of the 
body into different portions for the various vital functions, and 
by the centralization of the principle of life thus produced; 
and as they indicate, at the periods alluded to, the foundations 
of a higher collective organization, although one which has not 
attained its development in all the individual parts. Nature, 
it is true, is anxious to round off the sharpness of her divisions, 
but still the boundaries drawn by her are not the less fixed 
and determined. Thus we have neither intermediate forms 
nor a direct passage between insects and worms,* just as we 
have no intermediate steps between vertebrate and inverte- 
brate animals. 
The further subdivisions of the Linnean insects is-sufficient- 
ly distinct. The first class includes the insects taken in a 
more confined sense, with their division of the body into head, 
thorax, and abdomen, where the thorax alone possesses or- 
gans of motion, invariably six (or three pairs of) legs, and ge- 
nerally also wings, whose occurrence is only possible here, be- 
cause here alone a thorax exists, which is shut off from the 
head as well as from the abdominal part of the body. The 
second class, the Arachnide, are distinguished from the in- 
sects in this, that the head is blended with the thorax, and 
hence the constant want of feelers, and the conversion of the 
third pair of jaws into a fourth pair of feet, thus giving rise 
to eight legs. In both classes the abdominal or posterior part 
of the body is without organs of motion, which make their ap- 
pearance in the third class, the Crustacea. In the true Crus- 
tacea the mouth lies before the legs, and the pairs of feet 
which belong to the thorax become converted either wholly 
or in part into portions of the mouth; in the Entomostraca 
* Peripatus has been often instanced as a natural uniting link between 
the Myriapods and Annelides, but I must say that I haye never been able 
te find in it the slightest approach to an insect. 
