no Unconscious Memory- 

 creature has been accidentally turned over during its 

 journey, and reaches the top of tlie water-drop mth its 

 back uppermost, the vesicles will continue growing only 

 on one side, while they diminish on the other ; by this 

 means the shell is brought first into an oblique and then 

 into a vertical position, until one of the pseudopodia ob- 

 tains a footing and the whole turns over. From the 

 moment the animal has obtained foothold, the bladders 

 become immediatel}^ smaller, and after they have dis- 

 appeared the experiment may be repeated at pleasure. 



The positions of the protoplasm which the vesicles 

 fashion change continually ; only the grainless protoplasm 

 of the pseudopodia develops no air. After long and fruitless 

 efforts a manifest fatigue sets in ; the animal gives up the at- 

 tempt for a time, and resumes it after an interval of repose. 



Engelmann, the discoverer of these phenomena, says 

 (Pfliiger's Archiv fiir Physologie, Bd. II.) : " The changes 

 in volume in all the vesicles of the same animal are for the 

 most part synchronous, effected in the same manner, and 

 of like size. There are, however, not a few exceptions ; 

 it often happens that some of them increase or diminish 

 in volume much faster than others ; sometimes one may 

 increase while another diminishes ; all the changes, how- 

 ever, are throughout unquestionably intentional. The 

 object of the air- vesicles is to bring the animal into such 

 a position that it can take fast hold of something with its 

 pseudopodia. When this has been obtained, the air dis- 

 appears without our being able to discover any other 

 reason for its disappearance than the fact that it is no 

 longer needed. . . . If we bear these circumstances in 

 mind, we can almost always tell whether an arcella will 

 develop air-vesicles or no ; and if it has alread}^ developed 

 them, we can tell whether they will increase or diminish. 

 . . . The arcellcB, in fact, in this power of altering their 

 specific gravity possess a mechanism for raising themselves 

 to the top of the water, or lowering themselves to the 

 bottom at will. They use this not only in the abnormal 



