146 THE BEHAVIOR OF LOWER ORGANISMS. 



the anterior edge in a series of waves. The entire cylinder then rolls 

 forward just as the Amoeba does. 



The essential features of the movement seem to he (i) the advance of 

 the wave from the upper surface at the anterior edge ; (2) the pull exer- 

 cised by this wave on the remainder of the upper surface of the body, 

 bringing it forward. Most of the other phenomena follow as conse- 

 quences of these two. The flowing forward of the granules of the 

 endosarc seems to demand no special explanation, since a fluid con- 

 taining granules within a rolling sac must necessarily flow forward as 

 the sac rolls. By the movement forward of the anterior end a space is 

 left free ; by the rolling forward of the posterior end the fluid is piled 

 up and pressed upon, and must flow forward into the empty space in 

 front. Possibly there may be other causes at work in producing the 

 endosarcal currents, but such currents would be produced without 

 other cause in a sac moving as Amoeba does. 



FIG. 44. 



OTHKR SPECIES OF AMCEBA. 



Thus far we have dealt only with Amoebae of rather constant form, 

 which do not produce pseudopodia, or only rarely do so. We must now 

 take up species in which the form is changeable and the movements 

 varied. Of such species I have studied chiefly Amoeba Umax, A. 

 proteus, and a smaller Amoeba, which I take to be Amoeba angulata 

 Meresch. In these species the outer surface is not viscid, except at the 

 posterior end, so that small objects rarely cling to it. It is, therefore, 

 much more difficult to determine the direction of movement of the 

 upper surface than in Amoeba verrucosa and its relatives. Yet, by 

 mixing soot with the water, and devoting a sufficient amount of time 

 and patience to the work, one can obtain as many observations as he 

 desires. The soot settles upon the upper surface in particles or masses 



*Fio. 44. Side view (partly an optical section) of a creeping Amceba verru- 

 cosa, showing the thin anterior edge (A) attached to the substratum, and the 

 high posterior portion (P) with a cavity beneath it. 



t FIG. 45. Diagram of the movement at the anterior edge of Amoeba, verrucosa. 

 The region b-c pushes out, taking up the position b'-c' , and pulling forward the 

 region c-d, so that it comes to occupy the position c'-d'. The point a remains 

 in its place. 



