162 THE STUDY OF ANIMAL LIFE CHAP. 



surrounding the lower half of its body with a n ucus-like 

 sheath, the so-called tube. If a cloud of carmine particles 

 be introduced into the water-currents passing to the 

 ciliated mouth of the Stentor, it bends to the aboral 

 side, twisting on its stalk two or three times as it bends, 

 and thus it often succeeds in avoiding the falling particles. 

 This is answer one. But when the supply of carmine 

 particles is kept up, the ciliary movement is suddenly 

 reversed, and the water is driven away from the mouth. 

 This is sometimes repeated two or three times, and is 

 answer two. But if the Stentor does not get rid of the 

 obnoxious stimulation in either of these two ways, it 

 contracts into its tube and suspends activity. After half 

 a minute or so it re-expands, and if the carmine particles 

 still reach it, it contracts again. It will do this many 

 times and after each contraction it stays a little longer 

 in its tube than it did at first. This is answer three. 

 Finally, if no improvement in circumstances rewards the 

 Stentor's trials, it breaks from its attachment and swims 

 forwards or backwards away from its tube. This is 

 answer four. It will be seen that the animal tries a 

 series of reactions until by one of them it cets rid of 

 what was troubling it. " The phenomena," Jennings 

 writes, ' are thus similar to those shown in the ' learn- 

 ing ' of higher organisms, save that the modifications 

 depend upon less complex relations and last a shorter 

 time." 



5. Reflex Actions. When we pass from unicellular 

 to multicellular animals we find many illustrations of 

 what are called reflex actions. They resemble the 

 organic reactions already referred to, but are subtler and 

 depend upon structural predispositions of differentiated 

 nervous elements. The sea-anemone's tentacles close 

 upon the piece of flesh when it touches them ; the 

 nestling's mouth opens at the touch of th~> food in its 

 mother's beak ; the earthworm withdraws into its 

 burrow when it feels the tremor of a thrush's footstep ; 

 we cough in spite of ourselves when the crumb of bread 

 is going the wrong way, and so on. In typical and 

 simple cases, a reflex action involves (1) the " receptor ' 



