2 
ARTICULATED ANIMALS. 
dilations to produce the same variety of motion. From this 
also results a greater loss of force in the muscles, and conse- 
quently more general weakness in each animal, in proportion 
to its size. 
But the joints which compose the body have not always 
this sort of articulation : they are most usually united merely 
by flexible membranes, or emboxed one within the other; 
and then their motions are more varied, but do not possess 
the same force. 
The system of organs on which the articulated animals are 
most similar to each other, is that of the nerves. 
Their brain, situated on the oesophagus, and furnishing 
nerves to the parts which adhere to the head, is very small. 
Two cords, which embrace the oesophagus, are continued 
along the belly, uniting, from space to space, into double 
knots or ganglia, from which proceed the nerves of the body 
and of the limbs. Each of these ganglia performs the 
functions of a brain, for the surrounding parts, and suffices to 
preserve their sensibility for a certain time, when the animal 
has been divided. If we add to this, that the jaws of these 
animals, when they have any, are always lateral, and move 
from without to within, and not from top to bottom, and that 
there has not yet been discovered in any of them a distinct 
organ of smell, we shall have pretty nearly expressed all that 
may be said of them in general. But the existence of the 
organs of hearing ; the existence, number, and form of those 
of sight ; the product, and the mode of generation*; the nature 
of respiration; the existence of the organs of circulation; and 
even the colour of the blood, present great variations, which 
it will be necessary to study in the different sub-divisions. 
* A remarkable discovery on this subject is that of M. Herold, that in 
the egg of the Crustacea and arachnida, the vitellus communicates with the 
interior by the back. See his dissertation on the egg of spiders, Marburg, 
1824, and that of M. Rathke on the egg of the Astaci, Leipzig, 1829. 
