PHYLUM PROTOZOA. ONE-CELLED ANIMALS 



33 



Physiology 



An amoeba exhibits all activities necessary 

 to maintain itself, and which are characteris- 

 tic of higher animals. It moves about; cap- 

 tures, ingests, and digests food; egests undi- 

 gested matter; absorbs and assimilates the 

 products of digestion; secretes and excretes 

 various substances; respires; grows; repro- 

 duces itself; and responds to changes in its 

 environment. These facts indicate that the 

 amoeba is physiologically a very complex 

 organism. 



Amoeboid movement 



Amoebas move from place to place, cap- 

 ture other organisms, and ingest solid parti- 

 cles of food by means of fingerlike protru- 

 sions of the body known as pseudopodia 

 (singular pseudopodium). These pseudo- 

 podia may arise at any point on the surface 

 of the animal. The formation of the pseudo- 

 podium looks simple, but it has not yet been 

 explained with certainty in spite of detailed 

 investigations by some of our best zoologists. 

 When a pseudopodium is formed, a blunt 

 projection appears, which consists of ecto- 

 plasm. Granular endoplasm can be seen 

 flowing into this. The entire amoeba moves 

 forward in the direction of the pseudopo- 

 dium. Several pseudopodia may form at the 

 same time; usually one becomes large and 

 effective, and the others become smaller and 

 disappear. Actually, the amoeba moves along 

 by thrusting out pseudopodia and then flow- 

 ing into them. It has been observed to move 

 at the rate of one inch per hour, but the 

 rate of movement varies with the tempera- 

 ture, increasing up to a temperature of about 

 30° C, but ceasing at 33° C. 



Many cells in multicellular (metazoan) 

 animals, including man, exhibit typical 

 amoeboid movements. For example, the 

 white blood corpuscles in our own blood, 

 which are known as leukocytes, move from 

 place to place by means of pseudopodia and 

 are even able to work their way through the 



walls of blood vessels. Leukocytes also en- 

 gulf and destroy disease germs by means of 

 their pseudopodia, a process known as 

 phagocytosis. 



The two principal theories that have been 

 proposed to explain the formation of pseu- 

 dopodia are based on ( I ) changes in surface 

 tension, and (2) changes in the viscosity of 

 the cytoplasm. The subject is too complex 

 to be considered here in detail; further in- 

 formation can be obtained in advanced 

 books on zoology and in scientific journals. 

 There is still much to be learned about 

 amoeboid movement, but when the true ex- 

 planation is found it may give the key not 

 only to the formation of pseudopodia but 

 to the movement of flagella, cilia, and even 

 muscular contraction. 



Food 



The amoeba feeds principally on minute 

 animals and plants. Not every object en- 

 countered is ingested; a distinct selection of 

 food particles is evident (Fig. 12). It seems 

 rather surprising that the amoeba is able to 

 capture such rapidly swimming creatures as 

 the flagellate Chilomonas (Fig. 12A) and 

 ciliates such as the paramecium; the former, 

 however, is a favorite type of food. A Para- 

 mecium is sometimes held and actually cut 

 in two by the pseudopodia of the amoeba for 

 the purpose of ingestion. 



Ingestion 



Food may be engulfed at any point on the 

 surface of the body (see headpiece) of the 

 amoeba, but it is usually taken in at what 

 may be called the temporary anterior end, 

 that is, the part of the body extended to- 

 ward the direction of the animal's locomo- 

 tion. 



A food cup is usually formed in the follow- 

 ing way (Fig. 12D): pseudopodia enclose 

 the food particle from the sides; then thin 

 sheets of cytoplasm cover the top and the 

 bottom, thus entirely surrounding it. Often 

 when the prey is active, a large food cup is 



