LOCOMOTION IN AMCEBA 15 



all those actions or functions which are really essential for the 

 maintenance of life. 



In the higher animals the primary differentiation of the body 

 is into an outer protective and an inner digestive layer, each of 

 very complex structure. The Amoeba accomplishes the same 

 end in its own primitive manner by the differentiation into 

 ectoplasm and endoplasm. In most of the higher animals, 

 again, we find very well developed organs of locomotion in 

 the form of limbs. The Amoeba has no permanent organs of 

 locomotion at all but merely temporary projections of the body, 

 the so-called pseudopodia (Fig. 2, psd.), whicli are put forth when 

 required. In both cases, however, movement is effected in 

 essentially the same way, by contraction and expansion of the 

 living protoplasm. In the higher animals this power of con- 

 traction is localized in the muscles, which are highly specialized 

 for the purpose and have no other duties to perform, while in 

 the Amoeba any part of the body, or at any rate of the ectoplasm, 

 may contract or expand as occasion requires. 



In the process of formation of a new pseudopodium we see 

 first a thickening and protrusion of the clear ectoplasm (Fig. 2, 

 C., ect.), accompanied by a streaming in of the endoplasm, and 

 the latter seems to bulge out the ectoplasm as it flows forwards. 

 The pseudopodium is withdrawn again by a reversal of the pro- 

 cess, the endoplasm streaming out from it into the central mass 

 of cytoplasm and the ectoplasm contracting after the retreating 

 endoplasm. Thus at the posterior end of an actively creeping 

 Amoeba one frequently sees numerous blunt projections which are 

 the last remnants of retracted pseudopodia (Fig. 2, (7.). The 

 shape of the pseudopodia when fully extended differs very much 

 in different kinds of Amoebae. In some species they are com- 

 paratively short, thick and blunt, as in our illustration, while in 

 others they are very long and slender, radiating outwards from 

 the body of the animal in all directions. They are used by their 

 possessor not only as organs of locomotion but also as tactile 

 organs and, as we shall see directly, for the capture of prey. 



The protrusion and retraction of pseudopodia imply, of course, 

 the expenditure of energy, and this energy must be derived from 

 the combustion of the body of the Amoeba. In this way the 

 protoplasm is gradually used up, and, unless the animal is to die 

 of starvation, it must be replaced, which brings us to the 

 consideration of the manner in which an Amoeba performs the 



