BIOPSYCHOLOGICAL 635 



its cilia, backs away, turns slightly on its own axis, and then 

 swims ahead again. If it encounter the obnoxious stimulus a second 

 time, it repeats the process. This is the estabhshed reaction of the 

 Slipper Animalcule — the answer it gives to almost every question; 

 and in most cases it is very effective. 



In the behaviour of many Protozoa a given stimulus is followed 

 by a reaction which seems to depend on {a) the racial history, (6) the 

 specific metabolism of the animal, and (c) the individual experience. 



Thus in response to a particular excitation there may be pro- 

 trusion of an unusual form of pseudopodium, such as the delicate, 

 almost Radiolarian-like processes radiated out from the body of 

 an Amoeba when it is suspended in a large drop of water free from 

 contact with a surface. Perhaps the same kind of novel reaction is 

 seen among the cells of an embryo when they have been disordered, 

 or by phagocytes in the presence of novel stimuli. 



(X) TRIAL AND ERROR IN PROTOZOA.— Yet even at this uni- 

 cellular level there is something different — namely, a "trial and error" 

 testing of possibilities, the analogue of which may be recognised 

 among many-celled animals at various levels. A good example of 

 very simple "trial and error" procedure is afforded by the Trumpet 

 Animalcule, Stentor, a large ciliated Infusorian that attaches itself 

 by its narrow end to water-weed in marshy pools. It makes a 

 temporary mucus-like tube that invests the lower half of the body, 

 and into this it can retract itself. When Jennings allowed a shower 

 of microscopic carmine particles to sink upon the Stentor, he 

 observed that it may bend to one side, twisting on its stalk. This is 

 answer one. But if the shower of particles is kept on, the movement 

 of ciHa round about the upper end is suddenly reversed, and the 

 water is driven away from the mouth. The Stentor may repeat this 

 two or three times, and this is answer two. But if the dust continues 

 to fall the animal contracts into its tube and suspends activity. 

 After half a minute or so it re-expands, but contracts again if the 

 dust-shower continues. This retreat may be resorted to many times, 

 and after each contraction the Stentor stays a little longer in its 

 tube than it did at first. This is answer three. Finally, if no improve- 

 ment in circumstances rewards Stentor's trials, it breaks away from 

 its moorings and leaves its tube. This is answer four. We see, then, 

 that the animal tries a series of reactions until by one of them it 

 finds a way out of its difficulties. "The phenomena", Jennings 

 writes, "are thus similar to those shown in the 'learning' of higher 

 organisms, save that the modifications depend upon less complex 

 relations and last a shorter time." 



(XI) TENTATIVE BEHAVIOUR AMONG PROTOZOA.— But 



even at this low-lying level of animal activity there is the sugges- 



