ON THE FEEDING HABITS OF AMEBA 533 



also recorded in the following pages. This kind of discrimina- 

 tion which is exercised by the external protoplasm may be called, 

 for convenience in referring to it, histonic, in contradistinction to 

 organismal selection, in which the animal as a whole is concerned, 

 and which takes place before the substance is swallowed or re- 

 jected. As will appear later, it is very necessary that this dis- 

 tinction be clearly recognized, for so far as known, in no free 

 living omnivorous animal are the two terms synonymous. 



These few introductory remarks serve to make clear the prob- 

 lem of choice of food in animals generally; the following pages 

 are presented as a first study of this problem in ameba. 



MATERIAL 



The Amehas 



After a considerable amount of experimental work had been 

 done, it was discovered that the amebas described as Amoeba 

 proteus by Leidy ('79) and by Penard ('02) may be separated 

 into two groups as far as their behavior is concerned, and also 

 according to their general morphological aspect, as follows: 



The granular type. As the group name implies, these amebas 

 appear more or less densely granular, which appearance is caused 

 by the presence of large numbers of very small ovoid, crystalloid 

 bodies. Pseudopods are usually few in number and cylindrical 

 in shape. The direction of movement is not frequently changed. 

 A 'main' pseudopod may always be distinguished. They readily 

 eat dead organisms and small fragments of isolated proteins and 

 carmine. Small flagellates, such as chilomonas, are also fre- 

 quently eaten; indeed it is not improbable that this organism 

 forms the chief food supply of this type of ameba. Diatoms, 

 desmids, etc., are eaten very seldom. These amebas respond 

 only very slightly, in most cases not at all, to mild mechanical 

 stimulation, such as 'tickling' with a fine glass needle. 



Specimens of this type of ameba were obtained at various 

 places around Knoxville. During the summer and autumn, 

 cultures of this form were usually successful. They were made 

 as follows: Six or eight liters of dead leaves, water plants, etc.. 



