CHAPTER X. 



THE BIOLOGY OF AN ANIMAL (Continued). 



THE EARTHWORM. 



GENERAL PHYSIOLOGY. 



IN the preceding pages frequent mention has been made of 

 special physiological phenomena in connection with the detailed 

 descriptions of the primary functions and systems. It now re- 

 mains 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. 



Like the fern, the earthworm is a small mass of (mainly) liv- 

 ing matter occupying a definite position in space and time. It is 

 found in a certain environment for which it is very perfectly 

 fitted, and it is descended from a long series of ancestors in 

 which this adaptation has constantly been maintained by survival 

 of the fittest (compare p. 104). Harmony with the environment 

 appears both in the structure and in the habits of the animal. 

 For instance, the delicate integument gives to the body the flexi- 

 bility demanded by the peculiar mode of locomotion ; it affords 

 at the same time a highly favorable respiratory surface a matter 

 of no small importance to the worm in its badly-ventilated bur- 

 row ; and yet this delicate integument does not lead to desicca- 

 tion, because the animal lives always in contact with moist earth. 

 The alimentary canal, long and complicated, is most perfectly 

 fitted for working over and extracting nutriment from the earthy 

 diet. A striking instance of adaptation is shown in the care 

 which is taken to insure the welfare of the embryo -worms. 

 Minute, delicate, and helpless as they are, they develop in safety 

 inside the tough, leathery capsule (p. 146), floating in a milk-like 

 liquid which is at once their cradle and their food. 



The worm is no less perfectly adapted to its environment in 

 respect to habit. Its nocturnal habits protect it from heat, desic- 

 cation, from birds and other enemies; and even at night it keeps 

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