THE PROTOZOA 



133 





although apparently it holds the stage as well as any, and better than 

 most theories at this particular moment. The current seems to start at 

 the foremost part of the animal and extends backward. Jennings has 

 shown that Amoeba verrucosa resembles an elastic sac filled with fluid. 

 By placing this animal in a substance such as soot, which he caused to 



surround one of them, it was 

 shown that the streaming fol- 

 lowed the ectosarc toward the 

 forepart of the animal, and just 

 as it got beneath the Amoeba, 

 remained there until the ani- 

 mal had moved over it, when 

 it again moved upward at the 

 posterior end. 



Bellinger has shown that 

 whether on floor or ceiling, 

 wherever Amoebae are found 

 to move, there is a sort of 

 creeping walk by which one or 

 more outer parts of the animal 

 are extended at random. When 

 this projecting part comes in 

 contact with a solid substance 

 the most posterior attachment 

 relinquishes its pseudopod. It 

 is therefore assumed that there 

 is a contractile substance with- 

 in the animal. 



All the various experiments that have been performed along this 

 line have depended upon surface tension for their explanation. How- 

 ever, even if the animal moves in a similar manner to a drop of liquid 

 that is not living, it does not follow that it is the same force in each case 

 that causes the movement. 



It is .essential that all;of the subject headings under which the frog 

 was studied should also be borne in mind when the single-celled animals 

 come in for investigation. For example, in regard to metabolism the 

 following subjects must be studied just as in a more complex organism: 

 ingestion, digestion, eges-tion, absorption, circulation, assimilation, dis- 

 similation, secretion, excretion, and respiration. 



It can readily be understood that there must be some instinctive 

 process by which Amoebae know what food to ingest and what not to, 

 or they might continue to take in sand particles and indigestible sub- 

 stances which would cause the body to become so extended and heavy 

 that the animal would die from this effect alone. There are of course 

 no organs such as a mouth and intestinal tract as there are in the frog. 



Fig. 45. Locomotion of Amoeba proteus. 



Photographs in side view. A and B show a speci- 

 men attached at two points, a and 6, and a pseudopod 

 which projects from one end and bends down to the 

 substratum as in B at d ; C shows the extension of a 

 long pseudopod. (From Hegner after Bellinger.) 



