376 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



Some animal types have given rise to very small forms which are 

 able to pick out the little clusters of minute plants from the sea 

 water. It is rather curious that the two animal groups to which, 

 outside of the vertebrates, all the giants of the sea belong, the crus- 

 taceans and the mollusks, should have been the ones to produce the 

 vast bulk of small creatures which feed upon the little plants. 



Most numerous in kinds and numbers are the very small crus- 

 taceans of many different sorts which at certain places and at certain 

 seasons occur in myriads and are present in greater or lesser abun- 

 dance almost everywhere, and the very young stages of many of the 

 larger ones — crabs, lobsters, shrimps, etc. Just as on land the 

 insects are the chief intermediates through whose services plant sub- 

 stance is made available for spiders, scorpions, predaceous insects, 

 and most vertebrates that are not plant feeders, so their close rela- 

 tives, the crustaceans, are the main factors in the conversion of the 

 microsopic 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 important, 

 occurring in very great variety and in enormous numbers. Some- 

 times 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 whalebone whales. The euphausians also, small deli- 

 cate shrimplike creatures showing little variety, also feed on these 

 small plants, and may be as abundant in bulk, though not in numbers, 

 as the copepods. One of these, which in the springtime swarms in the 

 fjords of northwestern Europe, then forms the exclusive food of the 

 giant Blue whale in that region. The common rorqual, closely re- 

 lated 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 copepods and other small crusta- 

 ceans 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 mak- 

 ing 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, and 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. 



