EARTHWORM. 199 



surface with earth at the rate of 3 in. in fifteen 

 years. He was therefore led to the conclusion that earth- 

 worms have been the great soil-makers, or, more precisely, 

 that the formation of vegetable mould w r as mainly to be 

 placed to their credit. 



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 

 active 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 peregrina- 

 tions, 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 

 earthworm is halved with the spade, it does not necessarily 

 die, for the head portion may grow a new tail, while a 

 decapitated worm may even grow a new r head and brain. 

 Leucocytes help as usual in the regeneration. The earth- 

 worm is much persecuted by numerous enemies, e.g. centi- 

 pedes, moles, and birds. The male reproductive organs 

 are always infested by unicellular parasites Gregarines of 

 the genus Monocystis ; and minute thread-worms (Pclodera 

 pellio] usually occur in the nephridia and body cavity, and 

 often in the ventral blood vessels. 



Form and external characters. The earthworm is often 

 about 6 in. long, with a pointed head end, and a cylindrical body 

 rather flattened posteriorly. The successive rings seen on the surface 

 mark true segments. The mouth is overarched by a small lobe called 

 the prostomium, and the food canal terminates at the blunt posterior 

 end. The skin is covered by a thin transparent cuticle, traversed by 

 two sets of fine lines, which break up the light and produce a slight 

 iridescence. On a region extending from the 3 1st to the 38th ring, 

 the skin of mature worms is swollen and glandular, forming the 

 clitellum or saddle, which helps the worms as they unite in pairs, and 

 also forms the slimy stuff which hardens into cocoons. The middle 

 line of the back is marked by a special redness of the skin. On the 

 sides and ventral surface we feel and see four rows of tiny bristles 

 or seta:, which project from little sacs, are worked by muscles, and 



