i66 WORMS. 



and rain- drops ; the internal bruising reduces mineral matter 

 to more useful form ; while, in burying the surface with earth 

 brought up from beneath, the earthworms have been 

 ploughers before the plough. Darwin calculated that there 

 were on an average over 53,000 earthworms in an acre of 

 arable ground, that ten tons of soil per acre pass annually 

 through their bodies, and that they cover the surface with 

 earth at the rate of three inches in fifteen years. He was 

 therefore led to the conclusion, that earthworms have been 

 the great soil-makers, or more precisely, that the formation 

 of vegetable mould was mainly to be placed to their credit. 

 According to Gilbert White (1777), "the earth without 

 worms would soon become cold, hard bound, void of fer- 

 mentation, and consequently sterile;" while Darwin (1881) 

 said that " it may be doubted whether there are many other 

 animals which have played so important a part in the history 

 of the world, as have these lowly-organised creatures." 



Though without eyes, earthworms are sensitive to light 

 and persistently avoid it, remaining underground during the 

 day unless rain floods their burrows, and reserving their 

 public life for the night. Then, prompted by " love " and 

 hunger, they roam about on the surface, leaving on the 

 moist roadway the trails which we see in the morning. 

 More cautiously, however, they often remain with their tails 

 fixed in their holes, while with the rest of their body they 

 move slowly round and round. The nocturnal peregrin- 

 ations, the labour of eating and burrowing, the transport of 

 leaves to their holes, the collection of little stones to protect 

 the entrance to the burrows, include most of the activities 

 of earthworms, except as regards pairing and egg-laying, of 

 which something will afterwards be said. As digging is not 

 yet quite archaic, it may be noticed that when an earthworm 

 is halved with the spade it does not necessarily die, for the 

 head-portion may grow a new tail, while a decapitated worm 

 has even been known to grow a new head and brain. The 

 principal enemies of the earthworm are moles and birds, 

 while the male reproductive organs are always infested by 

 unicellular parasites — Gregarines of the genus Monocystis, 

 and little threadworms seem almost always to occur in the 

 excretory tubes. 



Form and External Characters. — The earthworm is often 



