OF ANIMALS. 101 



ficient quantity when plunged into water, and therefore perishes 

 of asphyxia. But at first sight, it is not so easy to explain wh} 

 an aquatic animal cannot continue to live when taken from th 

 water and placed in the air, for then we supply it with a fluid 

 richer in oxygen than that, the vivifying action of which was 

 sufficient for all its wants. There are, however, various circum 

 stances which, to a certain degree, explain this phenomenon. 

 Physics teach us that a body carefully weighed in air and in 

 water, is lighter in the last than in the first, and that, to sustain 

 it in equilibrium, there is then only required a weight equal to its 

 weight in air, less that of the bulk of water it displaces. Hence 

 it follows that animals whose tissues are too soft to sustain them- 

 selves in air, and are compressed to such an extent as to become 

 unfit to perform their functions in the organism, can nevertheless 

 live very well in water, where these same tissues, being not much 

 more dense than the surrounding fluid, are required to possess 

 only a feeble power of resistance to preserve their forms and to 

 prevent the several parts of the body from falling together on 

 each other. This consideration alone is sufficient to show us 

 why gelatinous animals, such as infusorise or medusss, are neces- 

 sarily inhabitants of the water; for, when we observe one of 

 these delicate creatures while still in this fluid, we perceive that 

 all the parts, even the most slender tissues, are sustained in their 

 proper position and float easily in the surrounding medium; but 

 the moment they are withdrawn, their body is almost entirely 

 effaced, offering to the eye only a confused and shapeless mass. 

 The influence of the density of the surrounding medium upon the 

 mechanical play of these instruments of life is also felt in ani- 

 mals of a more perfect structure, in which, however, respiration 

 is still carried on by means of ramified membranous appendages, 

 resembling diminutive shrub-branches or plumes. For example, 

 in anne'lidans or even in fishes, the branchise or gills are com- 

 posed of flexible filaments, which easily sustain themselves in 

 water, and therefore permit the respirable fluid to reach and 

 renew itself at all points of their surface; but, in air, these same 

 membranous filaments are in a measure effaced by their own 

 weight, falling one on another, and, in this way, exclude the 

 oxygen from the greater part of the respiratory apparatus. It 

 results that this function is then embarrassed, and the animal may 

 die of asphyxia in the air, although it found in water all it re 

 quired for free respiration. To convince ourselves of the impor 

 tance of these variations in the physical state of organs placed 

 in air or in water, it is only necessary to be reminded of what is 

 seen in dissecting-rooms : an anatomist desirous of studying the 

 structure of a very delicate part, would succeed very indifferently 

 if he made his dissection in air ; but by p.acing the subject of 



