METABOLISM IN THE SEA 191 



in the sea-bottom deposits, but we do not find 

 that these shell-fish feed to any extent on any 

 other form of animal life. These are only two 

 cases out of many which could be given, in each 

 of which inquiry into the foods and the foods of 

 the foods enables us to establish series of animal 

 forms linked together in such a way that one 

 regularly preys on the other. 



But it is not true that 



I 



" Large fleas have little fleas 

 Upon their backs to bite 'em ; 

 And little fleas have lesser fleas, 

 And so ad infinitum" 



Sooner or later each series of animal forms 

 comes to an end, and we find that the last, and 

 smallest, member does not prey on an animal 

 smaller than itself, but obtains its food by eating 

 some form of vegetable life, sea-weeds, diatoms or 

 other unicellular marine plants. In the sea^ as on 

 the land^ all animal life depends ultimately on vegetable 

 life for its sustenance. 



There is, in the long run, no essential morpho- 

 logical difference between an animal and a plant. 

 However obvious may be the differences in form 

 between a highly developed animal, such as a fish, 

 and such a relatively highly developed marine 

 plant as an alga, the corresponding differences 

 between the lower forms of animals, such as the 

 protozoa (say foraminifera and radiolaria), and the 

 unicellular plants like diatoms, are not apparent on 



