THE EARTHWORM 285 



ten and eleven, on the ventral side, are the openings of two pairs 

 of seminal receptacles in which the spermatozoa are stored after 

 being received from another worm during sexual union. Paired 

 nephridiopores open ventro-laterally from the nephiridia or 

 excretory organs, located in each segment except a few at the 

 anterior end. There are no organs of locomotion such as the legs 

 of many animals, since the worm moves by peristaltic extensions 

 and contractions of its body. There are, however, on each seg- 

 ment four pairs of minute spine-like setce, which function in loco- 

 motion by being extended at the proper angle to give the maxi- 

 mum hold upon a surface according to the direction of move- 

 ment (Fig. 137 B). 



General Activities. — The earthworm burrows in the earth and 

 is found from a few inches to several feet beneath the surface. 

 The burrows usually run straight down for several inches and then 

 wind about irregularly, sometimes reaching a depth of seven or 

 eight feet in soils of the right consistency. In winter the animals 

 are always found below the frost line and are relatively inactive. 

 Sometimes there is an enlargement at the lower end of the burrow 

 in which several worms may be found coiled up in knots during 

 this period of hibernation or winter rest. In loose soil the worms 

 burrow down by forcing the pointed anterior end between the par- 

 ticles of earth, as may be seen when they are thrown upon the 

 surface in the spading of a garden. In soil that is of normal con- 

 sistency, however, they excavate the burrow by literally eating 

 their way through the soil. The earth passes into the mouth and 

 through the digestive tract, and is deposited on the surface of the 

 ground as the faeces, or castings, which are always in evidence in 

 places where earthworms abound. This toilsome method of exca- 

 vation is not so wasteful as it might seem, because the soils that 

 are most suitable for these animals contain a considerable amount 

 of organic matter, which can be used as food, in addition to what 

 is secured at the surface of the ground. In this manner earth- 

 w^orms bring about important changes in the soil wherever they 

 are abundant. 



In his book entitled " Vegetable Mould and Earthworms," 

 Darwin showed how these animals, by constantly bringing to the 

 surface the lower layers of the soil and thus tending to bury 

 objects upon the surface, produce the same general effect as 

 that accomplished by man in cultivating a field by turning the 



