298 MOVEMENT IN LOWER ANIMALS 



Amoeboid movements. — In the study of the amoeba it 

 was noted that this animal moves by means of pseudo- 

 podia. These pseudopodia are merely extensions of the 

 body protoplasm into which the rest of the protoplasm 

 flowed. In other words, the amoeba extends its proto- 



FiG. 128 — Various shapes of an amceba in motion, 



plasm at a definite point on the sxirface, and then, by con- 

 traction, forces the rest of the protoplasm to flow in that 

 direction. The point where the pseudopods are pro- 

 duced varies with conditions and, since the animal has 

 no cell wall, theoretically these pseudopodia may be pro- 

 duced an3rwhere. In general, however, they are so formed 

 as to carry the animal toward food or away from irrita- 

 ting causes, and this fact shows that the expansions and 

 contractions are in some way a response to a stimulus. 



The result of this peculiar kind of contractility of proto- 

 plasm is to produce a slow creeping motion, and all animals 

 which move in this slow creeping fashion with the aid of 

 pseudopodia are said to show amoeboid movement. In 

 some of the forms of protozoans which have walls about 

 the body, amoeboid movement is still possible by extending 

 the pseudopodia through openings in the walls. The 

 little shelled form known as arcella and common in all 

 fresh water is an example of such a type. (See Fig 130.) 



In the higher forms we find no cases of amoeboid move- 

 ment on the part of the animals as a whole; but often the 

 cells of a part of the body may exliibit this method of 



