THE PROTOZOANS 



439 



Chap. 21 



Digestion, Absorption, and Assimilation. Amebas have no permanent re- 

 ception place for food. When an ameba first touches an inviting particle its 

 protoplasm rapidly flows around it and the food is engulfed as the ameba 

 passes over it (Fig. 21. 12). -Amebas never ingest dry food. Each bit is filmed 

 with water as our own food is cloaked with air or liquid. As soon as the food 

 is engulfed in the endoplasm, the digestive ferments flow into this temporary 

 stomach from the surrounding protoplasm. These digest the food, mainly the 

 proteins. The digested foods, the water and ferments are gradually absorbed 

 into the protoplasm. The indigestible remainders, such as diatom shells, stimu- 

 late the wall of the vacuole which squeezes them out of the body and all signs 

 of the food vacuole disappear. Finally the absorbed substances are assimilated, 

 arranged within their kindred materials in the living ameba. 



Respiration and Excretion. Oxygen held in the water diffuses through the 

 body of the ameba and unites with carbon and other substances within it. 

 This oxidation liberates energy and heat and leaves a by-product, carbon 

 dioxide, that either diffuses out through the body covering or collects in the 

 contractile vacuoles with other metabolic products, and is discharged with 

 them. 



The natural growth of the whole animal is a constructive process of change 



Fig. 21.12. "Pursuit, capture, and swallowing" of one ameba by another; escape 

 of the captured ameba and its recapture; final escape. The whole action took about 

 fifteen minutes. /, an ameba (a) from which the part b has been almost severed 

 by a glass rod; c is an ameba which has come in contact with part b and tries to 

 ingest it; 2-4. are stages in the ingestion, which are accomplished in 5 and 6 when 

 ameba a moves off and out of the story; 7-10 show that b is restless in the food 

 vacuole of c; at 7/ and 12, b escapes and moves away entirely out of contact with 

 c; 13, c pursues and captures b; for the second time b escapes, this time perma- 

 nently leaving c at 15 with temporarily vacant food vacuole. (Courtesy, Jennings: 

 Behavior of Lower Organisms. New York, Columbia Univ. Press, 1906.) 



