240 



ZOOLOGY 



Darwin's 

 experiments 

 with earth- 

 worms 



Structure of 

 earthworms 



from or excessively rare in the Rocky Mountain region ; 

 those found there today are cosmopolitan species intro- 

 duced by man, excepting a small form from the moun- 

 tains of Colorado. Generally speaking, the ocean is a 

 barrier to earthworms, and hence these, like amphib- 

 ians, are absent from oceanic islands, except when 

 introduced by human agencies. When islands are 

 found to possess many peculiar earthworms, we infer 

 that they were once united with the nearest continent. 

 The importance of earthworms to mankind has been 

 shown by Charles Darwin and others. In moist coun- 

 tries the ground may be seen to be almost covered with 

 their castings after a shower. They burrow through 

 the soil, and bring that which was below to the surface. 

 Darwin allowed a field to remain uncultivated for many 

 years, to see how soon and how deeply the worms would 

 bury objects originally left on the surface. The bury- 

 ing process is simply one of turning over the soil, 

 whereby the original surface is covered by material 

 from beneath, and eventually sinks. In this manner 

 the soil is subjected to the action of the bacteria which 

 work in the presence of oxygen and, breaking up in- 

 soluble chemical compounds, render the materials in it 

 fit for plant food. 



5. The so-called cocoons of earthworms are really 

 egg cases. They are oval or round objects composed of 

 chitin, containing several eggs. The oligochaetes are 

 hermaphroditic, possessing both male and female or- 

 gans, though these frequently mature at different times. 

 The alimentary canal is a straight tube running through 

 the body, not affected by the segmentation. Earth- 

 worms have no distinct organs of vision, but appear to 

 be sensitive to light. Although they cannot hear, they 

 readily appreciate vibrations in the soil. Unlike the 



