BEHAVIOUR OF SHORE ANIMALS 237 



This stereotropism or stereotaxis is essentially an orienta- 

 tion phenomenon and has its roots in the necessity for estab- 

 lishing a balance of contact among the nerve endings at the 

 surface of the skin. It supplements the other two modes 

 of orientation, viz. by means of light and gravitation. 

 Stereotropic phenomena are nowhere of more importance 

 than on the shore and one is led to speculate as to how 

 far shore conditions, with the influence of wave-shock and 

 the necessity for keeping a firm hold on the substratum, may 

 not have been instrumental in fostering this reaction. In 

 other words, bearing in mind Willey's remark on the appar- 

 ently fundamental nature of this reaction, is it too much to 

 ask whether this particular lesson may not have been learnt 

 on the shore ? 



Reactions to Gravity. — The upward growth of the plant 

 shoot and the growth of the root in a diametrically opposite 

 direction constitute the most obvious features of plant 

 behaviour. In normal conditions this behaviour of root and 

 stem remains unvaried and persists no matter in what way the 

 plant may be oriented. The root is said to be positively 

 geotropic, the shoot negatively geotropic. Behaviour of 

 an exactly similar kind has now been demonstrated by Loeb 

 and also by Miss Stevens for the hydroid Antenmilaria 

 antennina. This is perhaps not surprising, when we con- 

 sider the plant-like form of hydroids. The phenomenon, 

 however, according to Loeb, is not confined to sedentary 

 animals, but the burrowing and climbing movements of 

 worms and molluscs are of the same nature. Loeb justifies the 

 appUcation of the term " tropism " to the vertical movements 

 of some animals by assuming that gravitation acts by influenc- 

 ing the distribution of certain chemical substances in the 

 organism. When a plant or an animal is turned on its side 

 or inverted the distribution of these substances is altered, 

 and the physiological symmetry of the animal is destroyed. 

 If a specimen of Cerianthus (a sea-anemone) is placed head 

 downwards in a test tube filled with sea-water the tentacular 

 portion will gradually turn upwards, and the base will turn 

 downwards until eventually the normal orientation is reached 



