INTRODUCTION 

 The Structure and Mechanism of Fishes 



By WILLIAM K. GREGORY 



" When we no longer look at an organic being as a savage look* at a ship, as something 

 wholly beyond his comprehension; when we regard every production of nature as one which 

 has had a long history; when we contemplate every complex structure and instinct as the 

 summing up of many contrivances, each useful to the possessor, in the same way as any 

 great mechanical invention is the summing up of the labor, the experience, the reason, and 

 even the blunders of numerous workmen; when we thus view each organic being, how far 

 more interesting I speak from experience does the study of natural history become!" 



CHARLES DARWIN 



If Darwin's savage could see a U-boat rise to the surface, turn 

 around, submerge and rapidly move away, he would probably think 

 that the thing was alive and call it a great fish. He would certainly not 

 realize as we do that a submarine is only a complicated machine. On 

 the other hand when we look at a fish moving in the water, we are apt 

 to think of it only as a living thing and we scarcely realize that it is also 

 a mechanism, which although very complicated in its details is not diffi- 

 cult to understand in its main principles. We have become accustomed 

 to the thoughts that our own bodies are living machines which take in 

 fuel in the form of food, that we consume this fuel in a certain long, 

 winding chemical laboratory called the digestive system, and that we 

 use part of the energy derived from the food in moving from place to 

 place. Exactly as in ourselves the fish derives its motive power from the 

 slow combustion, or oxidation, of the fuel that it takes in as food; but 

 in the fish the necessary oxygen for consuming the food is extracted from 

 the water by the gills which thus serve the purpose of our lungs. 



In both fish and man the many-jointed backbone supports the body 

 and head. The fish, however, drives the body forward in a horizontal 

 position, while man carries his backbone vertically, standing on end as it 

 were, upon the legs, which are outgrowths of the body. In fishes the 

 legs are represented by the muscular and bony bases of the ventral fins 

 (see Plate 1), while their pectoral fins correspond to our arms. 



In both cases the body is propelled by muscles, which in the fish take 

 the form of rows of zigzag segments separated by thin membranes and 

 arranged along the sides of the body, while in man these primary 

 muscular elements are fused and combined into a complex system of 

 muscles for moving the limbs. 



In normal fishes the more or less fan-shaped tail is the chief pro- 

 pelling organ. It is attached to the backbone by V-shaped plates of 

 bone which grow downward from the lower side of the backbone (Fig 1). 



