CHAPTER XXI 



AN EARTHWORM 



I guess the pussy-willows now 

 Are creeping out on every bough 



Along the brook ; and robins look 

 For early worms behind the plough. 



Henry van Dyke, An Angler's Wish 



Habitat and Distribution. The animal (Lumbri'cus terres'- 

 tris) which is the subject of this chapter is distributed 

 widely throughout the world. This and very many other 

 species live in the soil, where their burrows extend obliquely 

 downward, sometimes many feet. One rather common spe- 

 cies, Helodri'lus foz'tidus, marked by red and dark bands of 

 color, lives in manure heaps. When irritated it gives off an 

 offensive odor, whence its specific name. The species most 

 commonly used in laboratories is Lumbricus terrestris. It 

 is found in small, cylindrical burrows, which it makes in the 

 soil wherever it is not too dry or too sandy. In spite of the 

 usual habit of living in the earth, these animals can live 

 for many days in water. Representatives of the many 

 species and the few genera of earthworms are found in 

 practically all places from Arctic to Antarctic regions, in- 

 cluding isolated oceanic islands. 



External Structure. In external structure the body of 

 an earthworm is very simple. The anterior end is slender 

 and pointed when extended in life, and the posterior region 

 is flattened above as well as below. The somites of the an- 

 terior region differ also from those in the posterior region in 

 being longer. There is no head, no thorax, and no abdomen. 

 There are no appendages except bristles, almost microscopic 



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