STRUCTURE OF EARTHWORM 



247 



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 w^hich 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 uni- 

 cellular parasites — Gregar- 

 ines of the genus Monocystis ; 

 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 flat- 

 tened posteriorly. The successive 

 rings seen on the surface mark true 

 segments. The mouth is overarched 

 by a small lobe called the pro- 

 stomium, and the food canal ter- 

 minates at the blunt posterior end. 

 The skin is covered by a thin trans- 

 parent cuticle, traversed by two 

 sets of fine lines, which break up 

 the light and produce a slight 

 iridescence. On a region extend- 

 ing from the 31st 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 

 setaj, 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 firmlv 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 

 surface there are not a few special apertures, which should be looked 

 for on a full-grown worm ; but careful examination of several specimens 



Fig. 134. — Anterior region of 

 earthworm. — After Hering. 



Note the eight setaj (s.) on each segment. 

 R.S., Spots between 9-10, lo-ii. 

 indicate openings of receptacula 

 seminis ; Ovd., openings of oviducts 

 on segment 14 ; y.^., openings of vasa 

 deferentia on segment 15. 



