THE DEEP SEA 187 



the fishes. The eggs of all our important food fishes, except the 

 herring, are found floating on the surface of the sea. 



THE DEEP SEA 



The fauna of the great abysses of the ocean is as interesting as 

 that of any other region, but it is removed from the direct scope 

 of Nature Study in school, and we shall not do more than hint 

 at the impressiveness of that strange, silent, cold, dark, plantless 

 world. There seems to be no depth limit to the distribution of 

 animals ; specimens have been fished up from a depth of over 

 five miles. Representatives of most of the types from Protozoa to 

 Fishes occur in the great depths, and the same or closely similar 

 forms are found over very wide areas, which indicates a 

 monotonous uniformity of physical conditions. 



The deep-sea world is in darkness, except in so far as it is 

 illumined by flashes of " phosphorescent " light ; a photographic 

 plate is not influenced below 250-500 fathoms. It is extremely 

 cold, about the freezing-point of fresh water, for the sun's heat 

 is virtually lost at about 150 fathoms, and there is little difference 

 between summer and winter. The pressure is enormous, at 2500 

 fathoms it is about 2 J tons per square inch ; it is quite calm, for 

 even the greatest storms are relatively shallow in their influence ; 

 there are no plants (except perhaps the resting phases of some 

 Algae), for typical vegetable life depends upon light; moreover, 

 even bacteria, otherwise almost omnipresent, are not known to 

 flourish in the great depths. The animals feed upon one another, 

 and ultimately upon the organic debris which sinks from above. 



FRESH WATER 



The number of different kinds of animals frequenting lakes, 

 ponds, rivers, marshes, and the like is small in comparison with 

 the sum of the marine list, but the fresh waters have not been 

 so much studied as the sea. The number of individual specimens 

 in a pond or lake is often enormous, e.g. in the case of water- 

 fleas, Rotifers, and Infusorians; but the number of species is 

 relatively small, and the number of distinct types still smaller. 

 The same forms occur in widely separated basins and in different 

 countries. Why is this ? 



