638 THE AMERICAN NA TURALIST. [Vol. XXXVI II. 



thread by the chloroform is not explicable in the manner sup- 

 posed by Rhumbler ; the surface tension of the drop has really 

 nothing to do with it. This is shown by the fact that such 

 a thread of shellac is coiled up in exactly the same manner if 

 submerged in a large vessel of chloroform, so that it is nowhere 

 in contact with the surface film. The coiling up is apparently 

 due to strains within the shellac filament, produced when it was 

 pulled out, and to the adhesiveness of its surface when wet with 

 chloroform. There are no corresponding factors in the Oscil- 

 laria thread ; this will indeed, as Rhumbler has shown, straighten 

 out again when released by the Amoeba. The process by which 

 Amoeba coils up the Oscillaria filament must thus be of an 

 essentially different character from that occurring in the experi- 

 ment. The explanation given by Rhumbler may of course still 

 be correct for the process in Amoeba, though it is not correct 

 for his imitation of the process. 



Amoeba does not ingest every small object with which it 

 comes in contact, but exercises an evident choice as to the 

 substances which it takes as food. Physical explanations and 

 imitations of such -choice have been given. We may notice 

 especially those set forth by the present author (:o2) in exten- 

 sion of certain experiments of Rhumbler. A drop of chloroform 

 is placed in the bottom of a watch-glass of water, and with fine 

 tweezers pieces of various substances are brought in contact 

 with its surface. Some are at once taken in ; others are not, 

 or are thrown out if forced into the drop. Glass, sand, dirt, 

 wood, gum Arabic, and chlorate of potash are rejected ; shellac, 

 parafiin, styrax, and hard Canada balsam are accepted. The 

 selection or rejection depends upon the relative amount of adhe- 

 sion between the solid object on the one hand and the chloro- 

 form and water on the other. Those which adhere more strongly 

 to the chloroform than to the water are taken in ; others are 

 rejected. 



These experiments show how choice might occur in an organ- 



they do 



show how it actually occurs in Amoeba. 



Food-taking is usually, as we have seen, not accompanied by 

 adhesion between Amc^ba and the food, so that choice of food 

 cannot be explained as due to the fact that some substances 

 adhere while others do not. 



