A SHORT LECTURE ON FISHES. 409 



backbone runs the spinal cord, that is the great nerve, which is 

 a continuation backward of the brain, protected by a bony tube, 

 and underneath it the great artery from the heart that supplies the 

 body with blood. To the sides of the backbone the ribs are all 

 attached, when present, and upon the spiny processes which pro- 

 ject upward is carried the framework of the fins of the back and the 

 tail. The side fins are supported upon the end of some fiat bones 

 hinged to the backbone, which answer to the fore-limbs in higher 

 animals, but the ventral or belly fins are fixed more simply. The 

 skeleton in fishes like the sharks and skates is very different, not 

 being bony at all, but composed entirely of cartilage ; hence they 

 have been named cartilaginous fishes. 



Most fishes swim with great rapidity, using their tail alone to 

 get ahead with, balancing and steering somewhat with their fins. 

 Many possess a very peculiar organ, called the swimming-bladder, 

 situated in the abdomen, this is a sac filled with air, and so ar- 

 ranged that the fish can increase or decrease his weight with 

 respect to the water (/. e., his specific gravity), and so sink or float 

 as he desires. 



The life of a fish is occupied wholly in providing its food and 

 escaping its enemies ; its senses are dull, and it seems to be with- 

 out any remarkable instinct. Yet it has a brain, abundant nerves, 

 and all the organs of sense. Fishes are great eaters, and nearly 

 all live upon flesh. Some have no teeth, but generally there are 

 large numbers in different parts of the mouth and throat, which 

 vary widely in form, number and position. The horrid set which 

 the shark carries is regularly shed and replaced by new. The 

 blood of fishes is red and cold, but it seems to circulate slower 

 than in warm-blooded animals, and the heart is more simple. 

 Some fishes, like the electric eel, have the remarkable power of 

 producing electricity. Fishes multiply by means of eggs, some 

 species producing hundreds of thousands at a single spawning — 

 as the dropping of the &gg is called ; yet perhaps not more than 

 one in a thousand of these eggs ever results in a full grown fish. 



Certain fishes change their residence with the seasons, or ac- 

 cording to their habits, especially their habit of spawning. This 

 migration in some is from north to south ; in others from deep to 

 shallow water and back again ; and many from the sea up rivers 



