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.) The rotation of a paramcecium 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. 
