370 ELEMENTARY BIOLOGY. [CHAP. 



by the thrusting out of lobe-like prolongations of the 

 peripheral part of the body, which are termed' pseudo- 

 podia, sometimes from one region and sometimes from 

 another. Occasionally, a particular region of the body is 

 constantly free from pseudopodia, and therefore forms its 

 hindmost part when it moves. Each pseudopodium is evi- 

 dently, at first, an extension of the denser clear substance 

 (ectosarc] only; but as it enlarges, the central, granular, 

 more fluid endosarc flows into its interior, often with a 

 sudden rush. 



The protoplasm is in some Amoeba crowded with small 

 watery vacuoles. In most there is present a clear space 

 which makes its appearance at intervals, in a particular 

 region of the ectosarc, and then disappears by the rapid 

 approximation of its walls. After a while, a small clear 

 speck or a line appears at the same spot and slowly dilates 

 until it attains its full size, when it again rapidly disappears 

 as before. Sometimes two or three small clear spots arise 

 close together, and run into one another to form the single 

 large cavity. The structure thus described is termed the 

 contractile vesicle or vacuole, and its rhythmical systole and 

 diastole often succeed one another with great regularity. 

 There is reason to think that it may be excretory like the 

 corresponding structure in Vorticella. 



One part of the Amoeba generally exhibits a rounded or 

 oval nucleus, while a still smaller body of about the ^ part 

 of the diameter of the nucleus, which may possibly be 

 compared to the para- or micro-nucleus of certain other 

 unicellular organisms, may be present (cf. p. 374). 



The gelatinous body of the Amoeba is not as a rule 

 bounded by anything that can be properly termed a 

 cuticular membrane ; all that can be said is, that its ex- 

 ternal or limitary layer is, in most cases, of a somewhat 



