The Living Machine 305 



isms. Thus a one per cent solution of morphine attracts 

 certain bacteria even though it is fatal to them. This is an 

 unusual condition however as morphine is a substance not 

 encountered in nature by bacteria. A similar behavior is 

 to be found, as we shall see later, in higher animals. Nature 

 sometimes plays the role of the enchantress Circe with the 

 humblest, as well as the proudest of her creatures. 



A step higher on the stage of life we come to Paramoecium, 

 whose acquaintance we have already been privileged to make. 

 Here we have an animal with definite organs of locomotion 

 (cilia) arranged in definite (spiral) lines upon the body; an 

 oral groove or food trough, leading to a gullet or primitive 

 digestive tract, a definite anal spot for the discharge of 

 undigested materials, specialized organs (contractile vacuoles) 

 for excretion, and specialized nuclei which play a complicated 

 role in the processes of metabolism and reproduction. Para- 

 moecium swims in a spiral path, directed by the spirally ar- 

 ranged cilia, and oblique oral groove. Its active movement 

 and peculiar form have caused many an unhappy hour to 

 the tyro in biology. If one place a drop of weak acid in the 

 dish of water in which Paramoecia are restlessly zig-zagging 

 to and fro, they will be found after a time to have gathered 

 in the drop; while vice v&rscu a grain of salt will soon be sur- 

 rounded by a zone of water free from Paramoecia save for the 

 dead bodies of a few, which have ventured too near the fatal 

 spot and failed to extricate themselves therefrom, ere death 

 o'ertook them. 



How are these results accomplished? Are Paramoecia at- 

 tracted by, and do they swim into the drop of acid because 

 they "like" it? And, similarly, do they avoid the salt be- 

 cause they "know it is bad for them"? Let us follow their 

 maneuvers a little more closely. If a Paramoecium in swim- 

 ming at random through the water, happens to approach a 

 drop of acid it is not repelled by it, and hence goes into the 

 drop if its direction of movement happens to take it there; 

 once inside the drop however should it "attempt" to escape 

 it cannot do so, for when it approaches the water outside the 

 drop it is seemingly repelled by the latter, for it backs up, 

 turns on its tail and swims away. Thus it can enter the 

 drop but cannot leave it, and in a short time a large number 

 of Paramoecia may be trapped in this manner. This behavior 

 of Paramoecium has been likened by Jennings who described it, 

 to a sort of "trial and error" behavior, similar to that of 

 the dog who learns to open a gate by putting his paw on the 

 latch, as a result of numberless fruitless pawings, in an at- 

 tempt to escape from the yard in which he is penned up. 

 Loeb however sees in this behavior something yet more simple 



