AMEBA 65 



94. Metabolism. — The food of an ameba consists of minute aquatic 

 plants and animals, though it will attempt to engulf any organism which it 

 can surround. Small bits of organic matter are also taken in, but inor- 

 ganic particles seem to be rejected unless they are accidentally taken in 

 with a bit of food. 



Ingestion takes place (Fig. 6) with the formation of pseudopodia. 

 These envelop the food, which thus becomes completely inclosed in the 

 body. With the food is taken in a certain amount of water and so a food 

 vacuole is formed. Into this vacuole is secreted hydrochloric acid, which 

 gives to its contents an acid reaction. An enzyme which acts on proteins 

 is then added and the reaction becomes alkaline. No enzyme acting on 

 carbohydrates has been demonstrated, and the same must be said for one 

 acting on fats. As digestion proceeds, the digested and dissolved food is 

 passed into the fluid of the food vacuole, whence it is absorbed into the 

 surrounding protoplasm. It is then circulated to all parts of the body and 

 assimilated wherever needed. Dissimilation also takes place everywhere 

 in the body, the digestive secretions being passed into the food vacuoles 

 and the excretions into the contractile vacuole. This, contracting at 

 intervals, expels the excretions from the body, thus accomplishing elimina- 

 tion. After all that is digestible of the food has been removed from it and 

 absorbed, the undigestible residue, or feces, is permitted to pass out 

 through the wall of the body, the animal simply flowing away and leaving 

 it behind. This is egestion. 



Respiration takes place everywhere through the body surface, but 

 the carbon dioxide may also be expelled through the contractile vacuole. 

 A continuous supply of oxygen is necessary for the life of the animal. 

 If an excess of food above the immediate needs is taken in, growth 

 occurs by intussusception and the animal gradually increases in bulk. 

 Food may not be immediately assimilated and undoubtedly some of the 

 granules present in an ameba which is well fed represent unassimilated 

 or stored food. Some of the crystals seen in the animal appear to be 

 excretions. 



95. Locomotion. — ^Locomotion in the ameba is accomplished by means 

 of the pseudopodia, which are temporary locomotor structures. Many 

 theories have been given to account for the manner in which pseudopodia 

 are formed and the way in which they effect locomotion. Their formation 

 has been attributed to a lessened surface tension at the points where 

 they are formed or to an increased surface tension elsewhere. The 

 freshly formed pseudopodium has been described as adhering to the sub- 

 stratum, the force of adhesion becoming a maximum at the extreme tip. 

 However, the most careful and detailed description of locomotion in the 

 ameba has recently been furnished by Mast. According to him the body 

 of the moving ameba is divided into four parts: the plasmalemma, which 

 is very thin and elastic; the clearer ectoplasm; the plasmagel, which is the 



