387 
THIRD GREAT DIVISION OF THE ANIMAL KINGDOM, 
THE ARTICULATED ANIMALS. 
This third general type of organization is quite as strongly characterized as that 
of the Vertebrata. The skeleton is not internal, as in the latter : but is seldom 
altogether absent, as in the Mollusks, The articulated rings which encircle the body, 
and frequently the limbs, supply the place of skeleton — and being, in almost every 
instance, tolerably hard, furnish the necessary resisting fulcra to the muscles of loco- 
motion ; whence, as among the Vertebrates, we find that the several actions of stepping, 
running, leaping, swimming, and flying, are performed by them. There are also some 
families among them that are either footless, or have merely soft and membranous 
articulated limbs, by which they can at most crawl. This external position of their 
hard parts, with the muscles inward, reduces each articulation to the condition of a 
case, and only permits of two kinds of movements. When attached to the next arti- 
culation by a closed joint, as in the instance of the limbs, the only motion is by 
ginglymus, that is, in a single direction, so that numerous articulations are required to 
impart variety of action : and from this results a very great loss of power in the 
muscles, and consequently a general feebleness in the creature in proportion to its 
magnitude. The articulated pieces which compose the body frame-work, however, 
are not always thus connected ; being oftener united by flexible membranes only, 
which slide considerably one over another, and so allow of more varied movements, 
but not of the same force. 
The system of organs in which all Articulated Animals bear the nearest resemblance 
to each other, is that of the nerves. 
Their brain, placed over the oesophagus, and supplying nerves to the parts ad- 
jacent to the head, is very small. Two chords, which encircle the oesophagus, are 
continued along the abdomen, and are connected at intervals by double knots or 
ganglia, from which the nerves of the body and of the limbs are sent forth. Each of 
these ganglia seems to perform the functions of a brain to the adjoining parts, and 
continues for a certain time to confer sensibility on them, after the animal has been 
divided. If to this be added, that the jaws of these animals, whenever they have 
any, are invariably lateral, and open and shut outward and inward, and not upwards 
and downwards, and that in none of them has a distinct organ of smell yet been dis- 
covered, nearly all has been expressed which it seems can be stated of them generally: 
for the existence of organs of hearing ; the presence, number, and form of those of 
sight; the productiveness and mode of generation*; their kind of respiration; the ex- 
* A remarkable discovery connected -with this subject is that of I See his Dissertation on the Eggs of Spiders, Marbourg, 1824 ; and 
M. Herold, who found that in the egg of Crustaceans and Arach- that of M. Rathke on the Eggs of Crabs, Leipsic, 1829. 
Hides, the yolk communicates with the back through the interior. — I 
c c 2 
