1 68 THE BEHAVIOR OF LOWER ORGANISMS. 



takes place in a series of jerks, and there is a corresponding jerkiness 

 in the contractions of the posterior part of the body. When a large 

 amount of surface is freed at once there is a sudden forward rush of the 

 fluid portion of the Amoeba, with a striking contraction of the posterior 

 part of the body. Such a case as is shown in Fig. 56 is only an unusu- 

 ally strong contraction of this sort, due to the fact that the hinder part 

 on the body had remained locally attached longer than usual. 



As a result of this contraction, the ectosarc of the posterior part of 

 the body becomes thickened and wrinkled, or warty. The change from 

 a flat plate to a rounded form involves a decrease in the amount of exter- 

 nal surface, and as the amount of material in the surface layer is not at 

 once decreased, this layer is compelled to fold and become wrinkled and 

 warty. When this process is very pronounced we have produced at 

 the posterior end the wrinkled, warty appendage so often described. 

 Such a roughened structure may be produced in any part of the Amoeba 

 by rapid contraction, as we have seen above (p. 160). The rough, 

 warty appendage at the posterior end is the common product of all the 

 contractions which have taken place. 



In Amoeba verrucosa and its relatives the current forward on the upper 

 surface extends backward to the posterior end ; the outer surface of the 

 latter seems not markedly different in texture from that of the rest of 

 the body. In other species of Amoeba there is a greater difference 

 between the texture of the surface layer of the anterior part of the body 

 and that of the posterior end, and this may involve some differences in 

 the movements. Often, in even these species, the forward current 

 extends backward to the very posterior end ; particles on the under side 

 pass up over the posterior end and forward, just as in A. verrucosa 

 (see p. 147). But in other cases, in A. Umax, A. proteus, etc., the 

 surface material at the posterior end is so stiffened that it is temporarily 

 excluded from the current. There is then produced the distinct, rough- 

 ened appendage, which is for a time dragged passively behind the 

 Amoeba. In such a case the currents from beneath pass upward on 

 either side of this appendage, meeting in the middle line (Fig. 57). 

 Particles attached to the under surface on either side of the appendage, 

 therefore, soon pass to the upper surface and are carried forward, while 

 those on the under surface of the appendage itself may remain in position 

 and be dragged forward for a considerable time. 



But I have rarely found this posterior appendage so completely cut 

 off from the general circulation as is often supposed. Usually there is 

 a very slow current forward on the upper surface of the appendage, 

 involving also its internal parts. Into this current particles attached 

 to the posterior end, and even to the under surface of the appendage, 

 are in course of time drawn. I have thus seen particles of soot dragged 



