420 APPLIED BIOLOGY 



The internal pressure of gases balances the enormous external 

 pressure; but if brought to the surface, they are killed by 

 the sudden expansion of internal gases. 



The spawning habits of some fishes are most remarkable. 

 Some salmon ascend the Columbia River from the sea for 

 more than a thousand miles, at an average rate of 3 to 4 

 miles a day. After depositing eggs and sperm-cells, they all 

 die; and no old fish lives to lead the young ones down 

 stream to the sea. This single spawning occurs after the 

 salmon are at least three years old. It is not true, as some- 

 times stated, that they always go back to the river where 

 they were hatched ; but they probably do not go far from the 

 mouth of the river, and hence are likely to ascend the same 

 river when fully developed and ready to spawn. 



The shad of our Atlantic coast is another example of 

 a fish that ascends rivers to spawn; but this fish lives to 

 return to the sea after spawning. 



The common river eels migrate downstream in the fall to 

 spawn in the sea, and after spawning in deep water, the old 

 eels die. Hence adult eels never migrate upstream. In 

 spring, vast numbers of young eels, about one year old, 

 appear below dams and waterfalls. A female 32 inches long 

 may have more than ten million eggs. The life-history of 

 eels was a complete riddle until about twenty years ago, 

 when it was found that the eggs are laid in deep sea-water. 



347. Types of Fishes. There are four distinct types of 

 fishes. Specimens should be examined, if possible. 



(1) The hag-fishes and lampreys are distinguished from 

 other fishes by the sucker-like mouth by which they attach 

 themselves to other fishes. The hag-fishes are even able to 

 bore into the bodies of their hosts. The lampreys are found 

 in lakes, rivers, and seas of temperate regions. On the coast 

 of California a species of hag-fish causes much damage by 

 taking baited hooks, entangling fishermen's lines, and boring 

 into captured fish. On the Atlantic coast they are not so 



