34 AMEBOID MOVEMENT 



locomotion but of position; they are not dynamic but static struc- 

 tures. But there are no hard and fast distinctions to be made 

 between these two types of pseudopods, for at least in bigemma 

 and bilzi, there are transitional forms of pseudopods (Figure 



M). 



The formation of pseudopods and their character depends to 

 some extent upon the firmness and thickness of the ectoplasmic 

 layer ; and the character of the ectoplasm in turn depends largely 

 upon the consistency of the protoplasm as a whole. In the fol- 

 lowing representative list of amebas : limicola, villosa, dubia, 

 protcus, discoides, laureata, bigemma, bilzi, radiosa, sphaeronu- 

 cleosus, vcrrucosa, the given order indicates a progressively 

 thicker and firmer ectoplasm as one passes from limicola to ver-* 

 rucosa.. But from limicola to bilzi the number of pseudopods 

 directing locomotion increases from one to an average of about 

 twelve in dubia, and then falls gradually to one in bilzi and the 

 others beyond in the list. (See Figure 10.) Where the directive 

 pseudopods begin to disappear, the transitional appear, viz., in 

 bigemma and bilzi; but beyond these no transitional pseudopods 

 occur. But along with the transitional there begin to appear also 

 the static pseudopods, which are seen relatively seldom in bi- 

 gemma and bilzi while in radiosa they occur at almost all times. 

 In sphaeronucleosus and verrucosa no distinctive pseudopods of 

 any kind occur. 



If all the known species of amebas in which the necessary 

 characteristics have been recorded, were arranged similarly with 

 respect to the firmness and the thickness of the ectoplasm, the 

 general relations of the various kinds of psendopods in the list 

 would be approximately the same as in the list given above ; but 

 there would appear an exception here and there, indicating the 

 operation of special factors. Such an exception, for example, is 

 seen in proteus in the list of species given, which because of the 

 ridges that it forms (Figure 3) has a smaller number of pseudo- 

 pods than would be the case if no ridges were formed 2 . It may 



2 This is shown by the fact that after this ameba has taken on a spherical 

 shape due to some disturbance in the water, the number of small ridgeless 

 pseudopods thrown out upon resuming movement, is about the same as 

 in dubia; but after ridges begin to form, the number of pseudopods de- 

 creases. 



