CHOICE OF FOOD IN AMEBA 237 



extremely crude; in fact it can hardly be said to exist at all 

 under ordinary conditions. 



But even if Paramecium discriminated between digestible and 

 indigestible substances, the supposition that it did so on a 

 chemical basis is open to question. For it is assuming a great 

 deal when it is said that Paramecium becomes aware in some 

 way of the diverse chemical structure of starch, milk, various 

 bacteria and bacilli, leukocytes, yeast cells, egg yolk, olive oil, 

 albumins, etc., while these substances are passed back to the 

 mouth by the cilia of the gullet. Now in order that Paramecium 

 may become aware of the chemical nature of these several 

 substances, new compounds must be formed in the sense organs 

 of the Paramecium (the cilia?) involving a permanent or tem- 

 porary union of a part or the whole of the molecule of starch, 

 albumin, oil, etc., with some structure in the sense organ, to 

 form, in each case, a definite and characteristic compound 

 which would then serve in some way to set into operation the 

 ingesting mechanism. What the chemical equipment of a sense 

 organ would have to be in order that compounds might be thus 

 formed with all the different substances which Paramecium eats, 

 it is difficult to conceive. Metalnikow does not attempt to 

 give an explanation of how selection on a chemical basis might 

 operate; nor to my knowledge do any of the other writers on 

 protozoa, although several refer more or less casually to the 

 selection of food as based on its chemical nature. That it is 

 unnecessary to explain choice of food upon a chemical basis 

 in Paramecium or in any of the other protozoa whose feeding 

 habits have thus far been investigated, will be shown by examining 

 the process of selection in ameba. 



Recently Lund ('14) published an important paper on the 

 feeding reactions of the large ciliate, bursaria. In this paper 

 Lund discusses the selection of food in bursaria, especially as 

 to whether it rests on a chemical or on a physical basis, and 

 in this connection refers to a previous paper of mine treating 

 of similar questions about the feeding behavior of stentor. It 

 seems however, that Lund did not quite understand my con- 

 ception of the difference between selection of food on a chemical 

 basis and on a physical basis, doubtless because I did not go 

 into the details of the difference as I conceive it. I have 

 examined his paper carefully but, although he insists that 



