THE EARTHWORM AND OTHER SEGMENTED WORMS 173 



in each segment, forming a ganglion from which nerves pass to 

 various parts of the body. 



Reproduction. - - Earthworms are hermaphroditic animals ; 

 that is, every individual is provided with both male and female 

 reproductive organs. When the time for egg laying approaches, 

 the eggs of the worm are inclosed in a cocoon which is secreted 

 by a glandular thickening of the body near the anterior end, the 

 clitellum. They are then fer- 



tilized by the spermatozoa 

 from another worm and de- 

 posited in the earth, where 

 they hatch into young worms, 

 resembling their parents ex- 

 cept in size. 



Economic Importance. - 

 Charles Darwin, in his book 

 on the Formation of Vegetable 

 Mold through the Action of 

 Worms, has shown, by careful 

 observations extending over 

 a period of forty years, how ^^ 



J J TIG. p8. -- Section through the upper 



great is the economic impor- stratum of a field showing the work of 



tanceofearthworms. Oneacre earthworms - 



of ground may contain over ear A thw ^< ; S^tSffiRiS 



fifty thousand earthworms, by worm castings ; D, subsoil not dis- 



The feces of these worms are SJjdL) 1 " 7 ^ earthworms ' (From 

 the little heaps of black earth, 



called " castings," which strew the ground, being especially 

 noticeable early in the morning. Darwin estimated that more 

 than eighteen tons of earthy castings may be carried to the sur- 

 face in a single year on one acre of ground, and in twenty years 

 a layer three inches thick would be transferred from the subsoil 

 to the surface. By this means objects are covered up in the 

 course of a few years. Darwin speaks of a stony field which 

 was so changed that " after thirty years (1871) a horse could 



