[3] THE FOOD OF MARINE ANIMALS. 487 



In the middle Atlantic Ocean large masses of algae (Sargassum hacci- 

 ferum) are floating about, which furnish food not only to the animals liv- 

 ing on and among them, but also, after they have decayed and sunk to 

 the bottom, to animals living in the depths of the sea. 



Plants growing on land likewise furnish food for the animals of the 

 sea. All rivers carry organic matter into the sea, which, with the fine 

 mineral substances of the river water, sink to the bottom near the mouths 

 of the rivers and form layers of rich mud. 



In the Caribbean Sea, A. Agassiz found masses of leaves, pieces of 

 bamboo, and sugar-cane at a depth of 900 la thorns, and at a distance of 

 16 to 24 kilometers from the shore, and wherever this was the case the 

 deep-water fauna was particularly rich. 



At the greatest depth of the ocean, below 900 fathoms, both the num- 

 ber of species of animals and individuals decrease, evidently because the 

 quantity of food is smaller. Decaying plants and animals which either 

 sink to the bottom perpendicularly or gradually glide down the incline 

 of the coast, are either devoured by marine animals or dissolve entirely 

 before they reach the deepest bottoms. 



Flat, sandy bottoms, on which the constant motion of the waves al- 

 lows nothing to rest, be it alive or dead, are almost void of animal life. 



Firm coral reefs, on the other hand, towards which wind and breakers 

 carry vegetable and animal life from the open sea, both by day and night, 

 are among the most densely inhabited portions of the sea, especially on 

 their outer edges, because these receive the greatest quantity of nutri- 

 tive matter (Murray). 



As the growth of young marine animals, just as much as that of land 

 and fresh-water animals, depends on the quantity of food, large numbers 

 of young fish and other marine animals, which were hatched within a 

 limited space, must spread over a larger extent of water, if many of them 

 are not to perish from want of food. To find this food, they swim in 

 large schools from one place to another, and continuing in the direction 

 in which they find the most food, they gradually get to migrating, with- 

 out the slightest idea or purpose of finding a more pleasant place of so- 

 journ. Thus the schools of herrings enter the bays of the Baltic, fol- 

 lowing those portions of the sea which are richest in copepods. The 

 herrings are followed by the codfish, which feed on the former, and near 

 the coasts of Norway large numbers of whales follow the herring and de- 

 vour many thousands (O. Sars). 



The migrations of marine animals are therefore caused by the periodical 

 appearance of food within certain parts of the sea, just as the migrations 

 of the South African antelope, the North American buffalo, and the Si 

 berian reindeer are produced by the same causes. 



Food-fish, by seeking their food in different parts of the sea, thus fur- 

 nish us with wholesome food from numberless small marine animals, 

 which without them would be of no use to us. 



