COMPARATIVE ANATOMY. 



Many animals of this order possess a 

 single or double swimming bladder; 

 which has been found in different in 

 stances to contain azote, hydrogen, and 

 oxygen. It has not been hitherto deter- 

 mined, whether it be subservient to any 

 other functions, besides that well known 

 one from which its name is derived. In 

 the mean time, like the air receptacles 

 of birds, it may be considered, without 

 impropriety, in the present division of the 

 work. 



It is placed in the abdomen, and close- 

 ly attached to the spine. It communi- 

 cates generally with the oesophagus, and 

 sometimes with the stomach, by a canal 

 (ductus pneumaticus,) containing in some 

 instances, as the carp, valves which seem 

 to allow the passage of air from the blad- 

 der, 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 or- 

 ders of both classes of these beings. But 

 in many cases direct proof has been ob- 

 tained on this point : experiment has ac- 

 tually proved the exchange of carbon for 

 oxygen. 



White-blooded animals are moreover 

 distinguished from those which have red 

 blood by this circumstance : that none of 

 the former, 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 par- 

 ticularly the land-insects, which consti- 

 tute, as is well known, by far the greatest 

 number of this class of animals, are fur- 

 nished with air-vessels, or tracheae, which 

 ramify over most of their body. These 

 tracheae are much larger and more nume- 

 rous 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 lar- 

 vae, and opens externally by nine aper- 

 tures (stigmata) : it produces on the in- 

 side the same number of trunks of air- 

 vessels (branchiae.) which are distribut- 

 ed over the body in innumerable ramifi- 

 cations. 



Both the tracheae and branchiae are of 

 a shining silvery colour ; and their princi- 

 pal membrane consists of spiral fibres. 

 The most numerous and minute ramifi- 



cations are distributed on the alimentary 

 canal. 



There is great variety in the number 

 and situation of the external openings, by 

 which insects receive their air. 



In most instances the stigmata are plac- 

 ed on both sides of the body. The at- 

 mospheric air enters by an opening at the 

 end of the abdomen in several aquatic 

 larvae, and even perfect insects. A very 

 remarkable change in this respect takes 

 place in several animals of this class dur- 

 ing their metamorphosis. Thus in the 

 larva of the common knats (culex pi- 

 piens,) 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 cfess of vermes, which compre- 

 hends such very different animals, 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 func- 

 tion, analogous to respiration, is carried 

 on in these animals, it must be effected 

 by methods which yet remain to be dis- 

 covered. 



Those vermes, however, which are 

 furnished with proper organs of respira- 

 tion, have the same variety in their struc- 

 ture which was remarked in insects. 

 Some, as the cuttle-fish, o\ ster, &.c. have 

 a species of gills, varying in structure in 

 different instances. But the greatest 

 number have air-vessels, or trachea. 

 Several of the testaceous vermes have 

 both kinds of respiratory organs, lu 

 some of the inhabitants of bivalve shells, 

 as the genus venus, the air-vessels lie be- 

 tween the membranes of a simple or 

 double tubular canal, found at the ante- 

 rior part of the animal, and capable of 

 voluntary extension and retraction. It 

 serves also for other purposes, as lay- 

 ing the eggs. The margins of i'ls 

 mouth are beset with the openings of 

 the tracheae. 



In the terrestrial gasteropodous mol- 

 lusca, 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 sides of the cavity'. 



ORGAN OF THE VOICE. 



Aristotle has correctly observed, that 

 those animals only which possess lungs, 

 consequently the three first classes of ths 



