THE INTERMEDIATE FOODS OF THE SEA l6l 



through whose services plant substance is made available for 

 spiders, scorpions, predaceous insects, and most vertebrates 

 that are not plant feeders, so their close relatives the crustaceans 

 are the main factors in the conversion of the microscopic plants 

 in the ocean into a form in which their substance can be used 

 by other invertebrates and by fish and whales. 



Of these little crustaceans, the copepods are the most im- 

 portant, occurring in very great variety and in enormous num- 

 bers. Sometimes they are so very abundant as to give a 

 pinkish or a red color to the sea for many miles, when they 

 become important as a food for certain whale-bone whales. 

 The euphausians also, small delicate shrimp-like creatures show- 

 ing little variety, also feed upon these small plants, and may 

 be as abundant, in bulk, though not in numbers, as the cope- 

 pods. One of these, which in the spring time swarms in the 

 fjords of northwestern Europe, then forms the exclusive food 

 of the giant Blue whale in that region. The common rorqual, 

 closely related to this monster and reaching a length of 70 feet, 

 feeds partly on fish and is frequently seen feasting among 

 shoals of herring which themselves are feeding upon the cope- 

 pods and other small crustaceans which consume the plants. 



Before leaving the copepods it should be mentioned that of 

 all sea creatures they have shown themselves the most versatile 

 in making use of reserves of food material. Besides the free 

 swimming ones, and the more numerous kinds of bottom living 

 ones, there are many that live in the food-collecting apparatus 

 of the sea-squirts stealing the food gathered by them, in the 

 digestive canal of crinoids, and in similar situations, while 

 others, become parasitic, and often very large, as "fish-lice," 

 prey upon the very creatures which, directly or indirectly, are 

 feeding upon their plant-eating relatives, just as the bird bot- 

 flies live on the blood of insect-eating birds. One of these, 

 in fresh water, lives upon the gums of the crocodile, which is 

 relieved of its unwelcome presence through the attentions of 

 the crocodile bird. 



Besides the very small crustaceans the chief plant eaters of 



