118 THE BIOLOGY OF AN ANIMAL. 



them into the burrow. Some of these are devoured ; the re- 

 mainder (including the pebbles, etc.) are used to line the upper 

 part of the burrow, and to plug up its opening when the worm 

 retires for the day. Besides bits of leaves and animal matter, 

 earthworms swallow large quantities of earth, which is passed 

 slowly through the alimentary canal, so that any nutritious sub- 

 stances contained in it may be digested and absorbed. This earth 

 is generally swallowed at a considerable distance below the sur- 

 face of the ground, and is finally voided at the surface near the 

 opening of the burrow. In this way arise the small piles of 

 earth (" castings" or faeces-} which every one has seen, especially 

 in the morning, wherever earthworms abound. Very large 

 quantities of earth are thus brought to the surface by earthworms 

 -in some cases, according to Darwin's estimates, more than 

 eighteen tons per acre in a single year. In fact, most soils are 

 continually being worked over by worms ; and Darwin has 

 shown that these humble creatures, in the course of centuries, 

 have helped to bury huge rocks and the ruins of ancient build- 



ings.* 



The earthworm has no ears, eyes, or any other organs of 

 special sense. Nevertheless and this is a point of great physio- 

 logical interest the fore end of the body is sensitive to light; 

 for if a strong light be suddenly flashed upon this part of the 

 worm as it lies stretched forth, it will often " dash like a rabbit 

 into its burrow." Of touch the animal has a keen sense, as may 

 be proved by tickling it; and its sense of taste must be well de- 

 veloped, since the worm is somewhat fastidious in its choice of 

 food. Earthworms appear to be quite deaf, but possess a dis- 

 tinct, though feeble, sense of smell. 



GENERAL MORPHOLOGY. 



Attention will first be directed to certain features of the body 

 seemingly of little importance, but really full of meaning when 

 compared with like features in other animals higher or lower in 

 the scale of organization. 



Antero - posterior Differentiation. The body (Fig. 72) has 

 an elongated cylindrical form, tapering to a blunt point at 



* Darwin, Ve.getfible Mould and Earthworms. Appleton, N. Y., 1882. ' 



