THE PROTOZOA 



based on the capacity of responsiveness, as shown by the reactions of a single 

 independent cell. The behavior of a multicellular animal is also based on the 

 responsiveness of cells, but such behavior involves reactions in sequence by 

 a number of cells. 



Most easily demonstrable reactions of amoebas are negative, since they 

 consist of withdrawals of pseudopodia or contraction of the cell in response to 

 stimulation. Certain other responses, to contact, for example, are positive 

 reactions. An amoeba dropped into water and settling slowly toward the 

 bottom through a considerable distance may give a positive response by ex- 

 tending pseudopodia in all directions (Fig. 8.3). If one of these pseudopodia 

 comes into contact with a surface, such as the stem of a water plant, the 

 amoeba may respond positively by flowing in the direction of this contact and 

 may thus begin to move over the surface. If one of its pseudopodia is then 

 touched with a glass needle, or if certain chemicals in solution are brought 

 into contact with a pseudopod by means of a capillary pipette, a negative 

 response may be indicated by withdrawal of the pseudopod. If the stimulus is 

 sufficiently strong, the entire amoeba may contract into a globular form. 

 Positive reactions are also involved in feeding, and these will be described 

 in the account of metabolism which follows. 



Feeding and Metabolism. Amoebas feed upon other organisms, both 

 animal and plant, and may thus be described as holozoic in their nutrition. 

 Such a species as Amoeba proteus is essentially a beast of prey, eating what- 

 ever it can capture, from small to relatively large protozoans and single- 

 celled plants. The most common food of this species consists of small 

 flagellates and ciliates, which an amoeba consumes in large numbers. Inges- 

 tion involves the extension of pseudopodia about the prey, which is engulfed 

 and transferred into the endoplasm (Fig. 8.4). A food vacuole thus originates 

 by the enclosure of a drop of water containing one or more food bodies. The 

 feeding reactions are surprisingly complex and variable, considering the ap- 

 parent simplicity of an amoeba. Forms such as motionless unicellular plants 

 e^'oke responses different from those induced by active prey. A certain selec- 



Fig. 8.4. Amoeba: ingestion of a flagellate and successive stages in the 

 formation of a food vacuole. (Adapted from W. A. Kepner and W. H. 

 Taliaferro 1913, Biological Bulletin, vol. 24, printed by permission.) 



233 



