280 Dr Erichson on the Classification of 
nourishment and propagation ; that is, the posterior part of 
the body from the anterior part, which latter alone contains 
the animal functions, inasmuch as it is provided with the or- 
gans of motion and those of the senses. The separation in 
insects is still more apparent and decided, when viewed in a 
narrower sense ; for we find the anterior part of the body di- 
vided into two parts, viz. the head, with the organs of sense, 
and the thorax, to which are exclusively confined the organs 
of motion. In the Arachnidz, the body is divided into only 
two parts, viz. anterior and posterior ; and here also, the first 
alone contains the organs of motion. In the Crustacea, each 
of the segments of the posterior part of the body has a similar 
pair of feet, as have the segments of the thorax; and thus the 
thorax in this class loses its original signification, and so much 
the more because the pairs of feet belonging to it, when those 
of the posterior part of the body eitherassume entirely or princi- 
pally the office of the progressive movement of the body (in the 
true Crustacea), at the same time become converted either 
wholly or in part into portions of the mouth. It follows, from 
the history of the metamorphosis of various Decapods, that no 
other type lies at the foundation of the structure of their bodies 
but that of insects, inasmuch as at a very early period of their 
existence no other legs except those of the thorax present them- 
selves, and which at that time exercise the functions of organs 
of motion. The Myriapods (Julus) also come into the world 
with three pairs of feet, and exactly those corresponding to 
the three pairs of feet occurring in insects. Inthe same man- 
ner, in the Lernaeadz, we find, for the most part, that they 
come out of the egg with three pairs of feet ; and it is only gra- 
dually that the posterior part of body, at first without feet and 
scarcely observable, is developed in a prominent manner. 
Another essential peculiarity of insects, considered in the 
Linnzan acceptation, is their possessing three pairs of jaws in 
the mouth, which, although variously modified, are always 
present. There is nothing analogous in the Linnzan worms, 
for neither the dental plates in the mouth of the leech, nor 
the hooks in the gullet of the Annelides, nor even the peculiar 
dental apparatus in the gullet of the Rotifera, have any re- 
semblance to the three pairs of jaws in insects. Although the 
