THE MOVEMENTS AND REACTIONS OF AMCEBA. 



167 



a. 



FIG. 56.* 



The contraction of the posterior part of the body is further shown by 

 the behavior of retracted pseudopodia. When a pseudopodium con- 

 tracts it usually produces, as we have seen, a 

 small wart-like excrescence, which persists 

 for some time. Such wart-like remains of 

 pseudopodia behave like foreign bodies at- 

 tached to the margin of the Amoeba. In the 

 anterior half they remain quiet, while in the 

 free posterior half they move slowly forward, 

 as a result of the contraction of this part of 

 the body. When two or more of these rem- 

 nants of pseudopodia are formed at once, 

 with an interval between them, this interval 

 becomes less as a result of the more rapid 

 movement of the hinder one. 



The contraction of this posterior region is 

 sometimes very striking, especially when the 

 posterior end becomes attached to some for- 

 eign object and is drawn out longer than 

 usual ; when it finally becomes free it con- 

 tracts suddenly and rapidly. Thus, for example, an Amoeba hav- 

 ing the form shown in Fig. 56, , began to move in the direction 

 shown by the arrow, when it became evident that the posterior end was 

 attached to a diatom shell, which was fast to the substratum. As the 

 Amoeba crept away the posterior end was drawn out, as shown at b. 

 Finally the diatom became detached from the bottom, when the stretched 

 posterior end at once contracted, shortening up rapidly, so that the 

 Amoeba had the form shown at c. Such observations are often made. 



This contraction does not occur in that part of the Amoeba which is 

 attached to the bottom, but begins at once as soon as the attachment 

 ceases. One might compare the outer layer to a stretched sheet of India 

 rubber that is attached to a surface by means of some adhesive sub- 

 stance. As soon as the adhesion gives way the sheet contracts. There 

 is no definite point at which the attachment to the substratum must 

 cease ; sometimes it is farther forward, sometimes farther back. The 

 gradual freeing of the posterior portion can be clearly observed in many 

 cases, particularly in Amoeba angulata, and may be seen to go hand 

 in hand with the contraction of the ectosarc. As soon as a certain part 

 of the body becomes free, its contraction takes place with some sudden- 

 ness, and the contraction is the more noticeable the greater the part of 

 the body that is freed at once. Often the process of becoming freed 



*FiG. 56. Successive forms of an Amoeba, showing the marked contraction 

 of the posterior end. (See text). 



