RESPIRATION. 239 



403. There is another point of view in which respiration 

 should be considered, namely, with reference to the buoyancy 

 of animals, or their power of rising in the atmosphere, and 

 their ability to live at different depths in the water, under a di- 

 minishedor increased pressure. The organs of respiration of birds 

 and insects are remarkably adapted for the purpose of admit- 

 ting at will a greater quantity of air into their body, birds being 

 provided with large pouches extending from the lungs into the 

 abdominal cavity and into the bones of the wing ; insects have 

 their whole body penetrated by air-tubes, the ramifications of 

 their tracheae, which are enlarged at intervals into wider cells, 

 whilst most of the aquatic animals are provided with minute, 

 almost microscopic tubes, penetrating from the surface into the 

 substance, or the cavities of the body for admitting water into 

 the interior, by which they thus adapt their whole system to 

 pressures which would otherwise crush them These tubes may 

 with propriety be called water-tubes. In fishes, they penetrate 

 through the bones of the head and shoulder, through skin 

 and scales, and communicate with the blood vessels and 

 heart, into which they pour water; in mollusca they are more 

 numerous in the fleshy parts, as, for example, in the foot, 

 which they help to distend, and communicate with the main 

 cavity of the body, supplying it also with liquid ; in echino- 

 derms they pass through the skin, and even through the hard 

 shell, whilst in polyps they perforate the walls of the general 

 cavity of the body, which they constantly fill with water. 



404. In order fully to appreciate the homologies between 

 the various respiratory apparatus observed in different animals, 

 it is necessary to resort to a strict comparison of the fundamen- 

 tal connections of these organs with the whole system of or- 

 ganization, rather than to the consideration of their special 

 adaptation to the elements in which they live. In vertebrata, 

 for instance, there are two sets of distinct respiratory organs, 

 more or less developed at different periods of life, or in dif- 

 ferent groups. All vertebrata, at first, have gills arising from 

 the sides of the head, and directly supplied with blood from 

 the heart ; but these gills are the essential organs of respira- 

 tion only in fishes and some reptiles, and gradually disappear in 

 the higher reptiles, as well as in birds and mammalia, towards 

 the close of their embryonic life ( 489). Again, all ver- 

 tebrata have lungs opening in or noar the head ; but the lun-s 



