Tropisms and Instincts as Adaptations 395 



are protected in other ways, so that they will not be eaten by 

 fish, probably owing to a bad taste ; so that it is not so appar- 

 ent that they are in real need of the protection that their 

 heliotropic response brings to them. Their turning towards 

 their food is, however, beyond question of great advantage to 

 them, for in this way they can find food that they cannot 

 detect in any other way. 



The unicellular plants were amongst the first organisms 

 whose tropic responses were studied, and the classical work 

 of Strasburger gave the impetus to much of the later work. 

 In recent years the unicellular animals, the protozoans, 

 have been carefully studied, more especially by Jennings. 

 His results show that the reactions in these animals 

 are different in some important respects from those met 

 with in higher forms. For instance, most of the free- 

 swimming infusoria are unsymmetrical, as are also many of 

 the flagellate forms, and as they move forward they rotate 

 freely on a longitudinal axis. It is therefore impossible that 

 they could orientate themselves as do the higher animals that 

 have been described above, and we should not expect these 

 Protozoa to react in the same way. In fact, Jennings shows 

 that they exhibit a different mode of response. Paramoecium 

 offers a typical case. As it moves forward it rotates toward 

 the aboral side of the body. As a result of the asymmetry 

 of the body, the path followed, as it revolves on its own axis, 

 is that of a spiral. Did the animal not rotate, as it swims 

 forward, its asymmetrical form would cause it to move in a 

 circle, but its rotation causes, as has been said, the course to 

 be that of a spiral, and the general direction of movement is 

 forward. 1 The rotation of a paramoecium on its axis is in turn 

 caused by the oblique stroke of the cilia that cover the sur- 

 face of the body. Their action when reversed causes the 

 animal to rotate backward. 



1 The same result is attained by a bullet that is caused by the rifling to rotate 

 as it moves forward. 



