I2O 



ANIMAL BIOLOGY 



objects tend slowly to settle down and eventually become 

 buried. Darwin observed a stony field which had thus 

 become changed "so that after thirty years a horse could 

 gallop over the compact turf from one end of the field 

 to the other, and not strike a single stone with its shoes." 

 Monuments and old buildings tend to settle slowly where 

 they are undermined by earthworms, and in time may be- 



PIG. 101. A tube-dwelling marine annelid. Note the branched gills 

 at the anterior end and the thread-like cirri by means of which the worm 

 entangles the small organisms that provide its food. (After Quatrefages.) 



come completely buried. In an average field Darwin 

 calculated that the amount of dirt carried to the surface 

 by worms in one year would form a uniform layer one- 

 fifth of an inch in thickness. Earthworms are thus con- 

 tinually plowing the ground, and although their operations 

 may seem slow they may effect great changes in the course 

 of centuries. 



Related to the earthworms, although having very dif- 

 ferent habits of life, is the group of annelids called leeches. 



