THE INTERMEDIATE FOODS OF THE SEA 1 63 



the open ocean are curious and delicate little molluscs, the ''sea 

 butterflies" or pteropods and their allies. But while some 

 forms of these eat plants, most of them live upon minute 

 plant eating animals, mostly small crustaceans and other 

 molluscs. There are, as compared with the crustaceans, rela- 

 tively few kinds; but some of them occur in incredible num- 

 bers, and in the seas about Greenland and in other places they 

 form an important part of the food of the whale-bone whales. 

 So abundant are some of the shell bearing species that in 

 various parts of the Gulf of Mexico, the Mediterranean, the 

 Bay of Biscay and elsewhere the sea bottom is more or less 

 exclusively composed of their dead remains, just as in other 

 places it is almost entirely composed of the shells of the foram- 

 inifera or the frustules of diatoms. 



The third method of devouring the minute oceanic plants 

 is by filtering the water and thus concentrating them. The 

 only oceanic animals that have recourse to this process are 

 the salps and the appendicularians, queer creatures allied to 

 the sea-squirts, and certain of the smaller fishes, Hke the men- 

 haden. Their straining apparatus is most wonderfully effi- 

 cient, and it is surprising to learn, from looking at the con- 

 tents of their stomachs through a microscope how small is 

 the size of some of the organisms they capture. 



In the bodies of the small crustaceans, the pteropods and 

 other allied molluscs, the salps and their relatives, the forami- 

 nifera and a few other types, the nutritive matter represented 

 by the microscopic plants is reassembled into units of appreci- 

 able size. Upon these units, for the most part upon the crus- 

 taceans which represent the most abundant and most generally 

 distributed of these units, feed all the other creatures of the 

 open ocean, directly or indirectly. 



Consuming these directly are larger crustaceans and molluscs, 

 numerous fishes, the herring and herring-like fishes, flying- 

 fishes, the young of all, or nearly all, other marine fishes, etc., 

 the larger salps, the jointed or annelidan worms, the nemer- 

 teans, the arrow-worms or chaetognaths, the whale-bone whales, 



