CHAPTER lY. 



THE BIOLOGY OF AN ANIMAL. 



The Common Earthworm. 



{Lumhricus terrestris, Linnaeus.) 



We now advance to a more precise examination of the living 

 body considered as an individual. It is a familiar fact that 

 living things fall into two great groups, known as plants and 

 animals. We shall therefore examine a representative of each 

 of these grand divisions of the living world, and inquire huw 

 they resemble each other and how they differ. Any liigher 

 animal would serve as a type, but the connnon earthworm is a 

 peculiarly favorable object of study, because of the sim})licity of 

 its structure, the clearness of its relation to other animals stand- 

 inty above and below it in the scale of orojanization, and the ease 

 with which it may be procured and dissected. Earthworms, of 

 which there are many kinds, are found in all parts of the world, 

 extending even to isolated oceanic islands. In the United States 

 there are several species, of which the most connnon are L. 

 commicnis {Allolobophora miccosa^ Eisen), L. terrestris^ and Z. 

 fmtichis {Allolopobliora fmtida^ Eisen). The tirst two of these 

 are found in the soil of gardens, etc., Z. terredv'is l)eing the 

 larger and stouter species and readily distinguishable by the 

 flattened shape of the posterior region, Z. fmtldu'^. a smaller 

 red species, transversely striped, and having a characteristic 

 odor, occurs in and about compost-heaps. 



Mode of Life, etc. Earthworms live in the earth, burrow- 

 ing through the soil at a depth varying from a few inches to 

 several feet. Here they pass the daytime, crawling out at 

 night or after a shower. The burrows proceed at iirst straight 



downwards, and then wind about irreorularlv, sometimes reacli- 



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