FISHES IN GENERA^ 



INTRODUCTION 



FISHES, defined in a popular way, are back-boned ani- 

 mals, gill-breathing, cold-blooded, and provided with fins. 

 It is in their conditions of living that they have differed 

 widely from the remaining groups of vertebrates. Aquatic 

 life has stamped them in a common mould and has pre- 

 scribed the laws which direct and limit their evolution ; it 

 has compressed their head, trunk, and tail into a spindle- 

 like form ; it has given them an easy and rapid motion, 

 enabling them to cleave the water like a rounded wedge. 

 It has made their mode of movement one of undulation, 

 causing the sides of the fish to contract rhythmically, 

 thrusting the animal forward. A clear idea of this mode 

 of motion is to be obtained from a series of photographs of 

 a swimming fish (Figs. 1-2) taken at successive instants : 

 thus in the case of the shark (Fig. i) the undulation of 

 the body may be traced from the head region backward, 

 passing along the sides of the body, and may be seen to 

 actually disappear at the tip of the tail. It is the press- 

 ure of the fish's body against the water enclosed in these 

 incurved places which causes the forward movement. 



The density of the living medium of fishes exerts upon 

 them a mechanical influence ; they are, so to say, balanced 

 in water, free to proceed in all planes of direction, poised 



