CHAPTER XII 



THE EARTHWORM: A STUDY IN SUBTERRESTRIAL 

 ORGANISMS 



There is every reason for iDelieving that the primitive organ- 

 isms lived in water, for we find the simpler organisms hving in 

 water to-day. Indeed, the small ancestral organisms would 

 dry up if exposed to air. Whether the very earliest organisms 

 developed in fresh water or in salt water is uncertain and im- 

 material, but it is quite certain that the principal branches of 

 the animal kingdom originated in the sea, on or near the short . 

 From the shallow seas their descendants passed on the one 

 hand into the depths of the sea and on the other either up the 

 estuaries and into the rivers or, in a few cases, directly upon 

 the land. Of the forms that thus came to inhabit fresh water 

 many have succeeded in reaching the shallow pools and small 

 streams which dry up during a part of the year. Such animals 

 have been forced, in order to protect themselves from desic- 

 cation, to penetrate into the muddy bottoms. This habit of 

 living in the ground is now common to many kinds of animals. 

 The elementarjr instincts with which the burrowing habit is 

 associated are, first of all, a love of contact and of darkness 

 and, usually, of moisture also. Tlic food of burrowing animals 

 is usuall.y any sort of organic debris and roots, but the higher 

 types, like the moles, feed on lower su})terrestrial organisms. 



Even at the seashore many kinds of marine animals have 

 acquired burrowing habits. Thus, many of the simplest 

 organisms, such as sponges, burrow into solid objects at the 



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