INTRODUCTION 17 



experiment. These have been found to be capable of 

 maintaining a horizontal course, or of ascending and 

 descending according to need. Their perception of the 

 upward and downward directions is, as experiment 

 shows, falsified by artificially induced changes in the 

 specific gravity of the medium, so that it is probable 

 that they are guided chiefly, if not altogether, by the_ 

 action of gravity. 



We have seen that air-breathing aquatic animals 

 are, as a rule, lighter than water, and that when they 

 come to rest they float upwards in one particular 

 position. The normal attitude is maintained by the 

 distribution within the body of two or more sub- 

 stances of very different density, such as air and 

 flesh or shell. Whatever the density of the medium, 

 and whether the body rises or falls, it will tend to 

 maintain the same position, the lighter tract tending 

 to rise. 



Ground-feeders, and swimmers which do not 

 breathe gaseous air, maintain the normal attitude in 

 a different way. Here the body is of nearly uniform 

 specific gravity, and the normal attitude is secured by 

 the movements of the limbs and the form of the 

 external surface. 



In ground-feeding aquatic animals, whose bodies 

 are a little heavier than water, the legs are usually 

 placed ventrally, and the back is rounded from side 

 to side. A Lobster is a familiar example. Here the 

 convex dorsal surface tends to move foremost, and 

 the action of the limbs treading the water, helps to 

 maintain the same direction of movement. As a 

 rule the limbs are turned down, and the body con- 



C 



