I'- 1 - ZOOLOGY. 



appropriate to itself at every instant a considerable quantity 

 uf oxygen, cannot find it in sufficient proportion when plunged 

 under water, and that then it must perish asphyxiated. But 

 at first view, it seems less easy to explain the causes by which 

 an aquatic animal cannot continue to live when withdrawn 

 from the water and placed in air, tor it is then furnished with 

 a liquid richer in oxygen than was the liquid, the vivifving 

 action of which sufficed for all its wants. There are, how- 

 ever, various circumstances which, to a certain point, explain 

 this phenomenon. Thus we learn by physics (natural philo- 

 sophy), that a body weighed successively in air and water, is 

 lighter in this latter than in the former, and that to maintain 

 it in equilibrium, a weight equivalent to that which repre- 

 sented its weight in air less that of the mass of water it has 

 displaced, is then sufficient. From this it results that ani- 

 mals whose tissues are too soft to support themselves in the 

 air, and which collapse to such a degree as to become unfit to 

 perform their functions in the organism, may yet live well in 

 the bosom of the waters where these same tissues, being 

 scarcely denser than the surrounding fluid, have occasion to 

 offer merely a feeble resistance to preserve their forms, and to 

 preserve the different parts of the body from collapsing on 

 themselves. This single consideration suffices to explain 

 why gelatinous animals, such as the infusoria and medusae, 

 are necessarily confined to the waters ; for when we observe 

 one of these delicate beings still plunged in this liquid, we 

 see that all its parts, even the most slender or delicate, sup- 

 port themselves in their normal position, and float with ease 

 in the surrounding medium ; but, so soon as we withdraw 

 them from it, their whole body collapses, and presents to the 

 eye merely a shapeless and confused mass. The influence of 

 the density of the surrounding medium on the mechanical 

 play of the instruments of life makes itself also felt on ani- 

 mals whose structure is more perfect, but in which, however, 

 respiration is performed by ramified membranous appendages, 

 like little bushes or bunches of feathers. Thus in the anne- 

 lides, or even in fishes, the branchiae or gills are composed of 

 flexible filaments, which support themselves easily in the 

 midst of water, and in this way permit the respirable fluid to 

 reach, and to be renewed at all points of their surface ; but in 

 the air these same membranous filaments collapse by the 

 effect of their own weight, fall on each other, and by that 

 alone exclude the oxygen from the greater part of the n^pira- 



