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CHAPTER VII. 



FISHES. 



In reviewing the series of animals which compose each 

 great division of this kingdom of nature, we constantly find 

 that the simplest structures and modes of progression are 

 those belonging to the aquatic tribes. Among vertebrated 

 animals, the lowest rank is occupied b}^ Fishes, a class com- 

 prehending an immense number of species, which are all 

 inhabitants of the water, which exhibit an endless variety 

 of forms, and open to the physiologist a wide field of in- 

 teresting research. We cannot fail to perceive, on the most 

 cursory glance, the beautiful adaptation of the form and struc- 

 ture of all these animals to the properties of the element in 

 which they are destined to reside. In order that the fish 

 might glide through the fluid with the least resistance, all 

 its vital organs have been collected into a small compass, 

 and the body has been reduced into the shape of a compact 

 oval, compressed laterally: and tapering to a thin edge, both 

 before and behind; for the purpose of readily cleaving the 

 water as the fish darts forward, and also of obviating the re- 

 tardation that might arise from the reflux of the water col- 

 lected behind. With a view^ to diminish friction as much 

 as possible, the surface of the body has been rendered smooth, 

 and the skin impregnated with oil, which defends it from 

 injurious impressions, and at the same time prevents the 

 water from penetrating into its substance. 



The body of a fish is nearly of the same specific gravity 

 as the water it inhabits; and the efiect of gravity is therefore 

 almost wholly counterbalanced by the buoyant force of that 

 fluid: for the weight of a mass of water, equal in bulk to the 

 body itself, is the exact measure of this buoyant force. If 

 this weight w^ere precisely the same as that of the fish, the 

 animal would be able to remain suspended in any pari of the 



