BEHAVIOUR OF SHORE ANIMALS 231 



body. But in spite of all such decline in direct sensitive- 

 ness to light there must have remained some trace of this 

 old primeval relationship to light." 



Many reactions to light almost certainly come under 

 the head of tropisms. That is to say they are obligatory 

 responses under the direct compulsion of an external 

 stimulus : as when a moth flies into the flame of a candle. 



The key to this type of behaviour lies in the symmetrical 

 structure of the animal, symmetrical not only morphologi- 

 cally, but physiologically as well, " inasmuch as under normal 

 conditions the chemical constitution and the velocity of 

 chemical reactions are the same for symmetrical elements 

 of the body," e.g. the sense organs (Loeb, 1918). When a 

 sense organ of one side is stimulated and not the corre- 

 sponding one of the other, as when light falls upon one of 

 the eyes, the physiological symmetry of the brain is disturbed, 

 and this disturbance being communicated to the muscles 

 of the more illumined side they are thrown into a state of 

 increased tension. The effect of this is to cause the animal 

 to turn head and body until both eyes are equally illuminated, 

 when, the physiological symmetry being restored, the animal 

 proceeds in a straight line towards the source of light. 

 The term tropism is best retained for the bending or 

 growth movements of sedentary organisms in response to 

 external stimuli, while we can apply the term taxis to the 

 method of orientation displayed by freely-moving organisms. 

 Thus we may speak of the heliotropism or phototropism 

 of hydroids and tube- worms, and the phototaxis of a Littorina 

 or larval barnacle. This makes a convenient method of 

 distinguishing between the responses of sedentary and 

 motile organisms. 



An important feature of tropisms is that they may be 

 modified or even completely reversed by changes in the 

 external medium, or in the internal physiological condition 

 of the animal. Again, an animal may be positively photo- 

 taxic at one stage of its existence and negatively so at another. 

 The responses vary with the nature of the habitat : thus, 

 Bohn (1908) finds that starfishes {Asterias ruhens) from 



