STRUCTURE OF THE EARTHWORM. 167 



about six incheslong, with a pointedhead end, and a cylindrical 

 body rather flattened posteriorly. The successive rings seen 

 on the surface mark true segments. The mouth is over-arched 

 by the most anterior (pre-oral) segment, while 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 irides- 

 cence. On a region extending 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 forms the slimy stuff which hardens into 

 cocoons for the eggs. 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 setse, 

 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 surface, there are 

 not a few special apertures, which should be looked for on 

 a full-grown worm, but careful examination of several speci- 

 mens is usually necessary. Almost always plain on the isth 

 ring are the two swollen lips of the male ducts, less distinct 

 on the 14th are the apertures of the oviducts through #hich 

 the eggs pass, while on each side between segments 9 and 

 10, lo and II, are the openings of two sperm-reservoirs 

 or spermathecae into which male elements from another earth- 

 worm pass, and from which they again pass out to fertilise 

 the eggs of the earthworm when these are laid. Each 

 segment contains a pair of excretory tubes, which have minute 

 ventral-lateral apertures, while on the middle line of the 

 back between every two adjacent rings there are minute 

 pores, through which fluid from the body-cavity may exude. 

 Skin and Bristles. — Outermost lies the thin cuticle, a 

 smooth shred of which should be mounted on a slide and 

 examined under high power, to see the intersecting lines 

 which produce interference of light and iridescence. Like 

 any other cuticle, it is produced by the cells which lie 

 beneath, and it is perforated by the apertures previously 

 mentioned. The epidermis clothing the worm is a single 



