2 1 2 PHYL UM ANNELIDA 



precisely, that the formation of vegetable mould was 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 pro- 

 tect 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 neces- 

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

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

 Phagocytes 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 j and minute thread-worms (Pelodera 

 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 prostomiuin, 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 tihe 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 

 setae, which project from little sacs, are worked by muscles, and assist 

 in locomotion. These bristles are fixed like pins into the ground, at 

 times so firmly that even a bird finds it difficult to pull the worm 

 from its hole. As each of the four longitudinal rows is double, there 

 are obviously eight bristles to each ring. On the skin of the ventral 



