Food Selfxtton Among Lower Animals. 35 



Selection of Food Among Lower 

 Animals 



By Asa A. Schaeffer, University of Tennessee. 



All animals must eat food in order to live. The higlier animals 

 usually do not eat anything that is not food, but some of the lower 

 forms, such as the earthworm, eat a considerable amount of ma- 

 terial that is not useful for food. In a general way it has been 

 assumed that the further we go down the scale the less precise is 

 discrimination between food and other materials. This idea has 

 in the past been responsible for the notion that discrimination in 

 food does not occur in the one-celled animals, for these stand at 

 the bottom of the scale. The expectation was, then, that these low 

 forms of animal life ate all the different kinds of particles that 

 they happened upon, whether of food value or not. This opinion 

 was expressed by some of our foremost students of the protozoa, 

 and their opinions have been generally held for a long time. 



It was with a view of determining the truth of this notion that 

 I carried out a number of experiments on several of these lower 

 forms. A considerable number of test substances were employed, 

 some of which were good for food, others not. The principal 

 question to be solved was : Can a one-celled animal tell the dif- 

 ference between food substances and those that are not good for 

 food? And if they can tell the difference, how can they tell it? 



I wish today to report on a few experiments on stentor and on 

 ameba that were designed to throw some light on these questions. 

 Both stentor and ameba are of about the same size, being just 

 barely visible to the naked eye, and live amid the same general sur- 

 roundings, in ponds and other bodies of fresh water. These organ- 

 isms are entirely different in structure. While each consists of 

 only a single cell with a single nucleus, their body structure is as 

 unlike as it can well be. 



