PRELIJUNARY OBSERVATIONS. 9 



whicli exhibit a wide range of structural 

 variation, and habits and instincts of the most 

 opposite character. 



Last in the series of animals with a brain 

 and spinal cord comes the vast variety of fishes. 

 These animals are expressly constructed for the 

 water in which they are destined to live, and 

 through which many dart along with almost 

 inconceivable velocity. They are furnished 

 with gills for the purpose of aquatic respiration ; 

 their bodies are generally somewhat cylindrical, 

 compressed at the sides, with a sharp pointed 

 head, and an elongated muscular tail, terminated 

 by a fin, which is the great instrument of 

 locomotion, the fins of the body acting chiefly 

 as rudders and balancers. However, there is 

 great difference in the form of fishes, and, 

 consequently, in the ease and celerity of their 

 movements, and in their habits. Some have 

 large heavy heads, as the cod-fish, and move 

 leisiu"ely — some are almost globular, as the 

 tetraodon, which has a habit of floating on the 

 siurface of the water — some, again, as the skate, 

 are depressed vertically, and are termed flat- 

 fishes — some are compressed laterally, with the 

 head twisted, or, as it were, distorted, so that 

 both eyes are on the same side, such is the 



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