i88 SEGMENTED WORMS OR ANNELIDA. 



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 fermentation, 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 peregrinations, 

 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. When an earth- 

 worm 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 earthworm is much persecuted by numerous enemies, 

 e.g., centipedes, moles, and birds. 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 about six inches long, with a pointed head 

 end, and a cylindrical body rather flattened posteriorly. The successive 



