COMPARATIVE ANATOMY. 
passage of air from the bladder, but not to 
admit its entrance from without. 
That white-blooded animals indispensa- 
bly require a species of respiration, would 
have been inferred by analogy from the 
wonderful apparatus of gills or tracheae, 
which have been discovered in most orders 
of both classes of these beings. But in 
many cases direct proof has been obtained 
on this point : experiment has actually 
proved the exchange of carbon for oxygen. 
White-blooded animals are moreover dis- 
tinguished from those which have red blood, 
by this circumstance ; that none of the for- 
mer, as far as we hitherto know, take in 
air through the mouth. 
Many aquatic insects as the genus can- 
cer have a species of gills near the attach- 
ment of their legs. The others, and parti- 
cularly the land-insects, which constitute, 
as is well known, by far the greatest num- 
ber of this class of animals, are furnished 
with air-vessels, or tracheae, which ramify 
over most of their body. These tracheae 
are much larger and more numerous in the 
larva state of such insects as undergo a 
metamorphosis, (in which state also the 
process of nutrition is carried on to the 
greatest extent) than after the last, or, as 
it is called, the perfect change has taken 
place. 
A large air-tube (trachea) lies under the 
skin on each side of the body of larv®, and 
.opens externally by nine apertures (stig- 
mata) : it produces on the inside the same 
number of trunks of air-vessels (branchi®,) 
which are distributed over the body in in- 
numerable ramifications. 
Both the trache® and branchi® are of a 
shining silveiy colour ; and their principal 
membrane consists of spiral fibres. The 
most numerous and minute ramifications 
are distributed on the alimentary canal. 
There is great variety in the' number 
and situation of the external openings, by 
which ins'ects receive tlieir air. 
In most instances the stigmata are placed 
on both sides of the body. Tlie atmosphe- 
ric air enters by an opening at the end of 
the abdomen in several aquatic larv®, and 
even perfect insects. A very remarkable 
change in tliis respect takes place in seve- 
ral animals of this class during their meta- 
piorphosis. Thus in the larva of the com- 
mon gnats (culex pipiens,) the air enters 
by an opening on the abdomen : while in 
the nympha of the same animal, it gains 
admission by two apertures on the head. 
In the class of vermes, which compre- 
liends such very different amma.s, the 
structure of the respiratory organs is pro- 
portionally various. Some orders, as those 
which inhabit corals, the proper zoophytes, 
and perhaps the intestinal worms, appear 
to be entirely destitute of these organs ; so 
that if any vital function, analogous to res- 
piration, is carried ou in these animals, it 
must be effected by methods which yet re-, 
main to be discovered. 
Those vermes, however, which are fur- 
nished with proper organs of respiration, 
have the same variety in their structure, 
which was remarked in insects. Some, as 
the cuttle-fish, oyster, &c. have a species 
of gills, varying in structure in different 
instances. But the greatest number have 
air-vessels or trache® . Several of the testa- 
ceous vermes have both kinds of respiratory 
organs. In some of the inhabitants of bi- 
valve shells, as the genus venus, the air- 
vessels lie between the membranes of a 
simple or double tubular canal, found at 
the anterior part of the animal, and capable 
of voluntary extension and retraction. It 
serves also for other purposes, as laying 
the eggs. The margins of its mouth are 
beset with the openings of the trache®'. 
In the terrestrial gasteropodous mollusca, 
of which we may instance the snail and 
slug, there is a cavity in the neck receiving 
air by a small aperture, which can be 
opened or shut at the will of the animal. 
The pulmonary vessels ramify on the sidea 
of the cavity. 
ORGAN OF THE VOICE. 
Aristotle has correctly observed, that 
those animals only which possess lungs, con- 
sequently the three first classes of the ani- 
mal kingdom possess a true voice. Several 
genera and species even of these are either 
entirely dumb, as the anteater, the manis, 
the cetacea, the genus testudo, several 
lizards and serpents; or they lose their 
voice in certain parts of the earth, as the 
dog in some countries of America, and quails 
and frogs in several parts of Siberia. 
Most mammalia have the following cir- 
cumstances in common : their rima glottidls 
is provided with an epiglottis, which in 
most instances has a peculiar muscle, arising 
from the os hyoides, and not found in the 
human subject : the margins of this rima 
are formed by the double ligamenta glotti- 
dis (ligamenta thy reoary tanoidea) ; between 
which the ventriculi laryngis are formed. 
The epiglottis does not exist in most of the 
bat kind : and in some mouse-like animals, 
