CHAPTEE YII. 



THE BIOLOGY OF AN ANIMAL (Continued.) 

 Physiology of the Earthworm. 



IN the preceding pages brief descriptions of many special 

 physiological phenomena have been given in connection with the 

 detailed descriptions of the primary functions and systems. It 

 now remains to consider the more general problems of the life of 

 the animal, and especially its relations to the environment, and 

 the transformations of matter and energy which it effects. 



The Earthworm and its Environment. The earthworm is an 

 organized mass of living matter occupying a definite position in 

 space and time, and existing amid certain definite and character- 

 istic physical surroundings which constitute its ' 6 environment. ' y 



As ordinarily understood the term environment applies only 

 to the immediate surroundings of the animal to the earth 

 through which it burrows, the air and moisture that bathe its 

 surface, and the like. Strictly speaking, however, the environ- 

 ment includes everything that may in any manner act upon the 

 organism that is, the whole universe outside the worm. For 

 the animal is directly and profoundly affected by rays of light 

 and heat that travel to it from the sun ; it is extremely sensitive 

 to the alternations of day and night, and the seasons of the year ; 

 it is acted on by gravity; and to all these, as well as to more 

 immediate influences, the animal makes definite responses. 



We have seen that the body of the earthworm is a compli- 

 cated piece of mechanism constructed to perform certain definite 

 actions. But every one of these actions is in one way or an- 

 other dependent upon the environment and directly or indirectly 

 relates to it. At every moment of its existence the organism is 

 acted on by its environment ; at every moment it reacts upon 

 the environment, maintaining with it a constantly shifting state 

 of equilibrium which finally gives way only when the life of the 

 animal draws to a close. 



Adaptation of the Organism to its Environment. In its rela- 

 tions to the environment the earthworm embodies a fundamental 



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