STRUCTURE OF THE EARTHWORM. 



189 



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 irridescence. On a region extending from 

 the 3 1st to the 38th ring, the skin of mature worms is swollen and 

 glandular, forming the clitellnm or saddle, which helps the worms as 

 they unite in pairs, and perhaps forms the slimy stuff which hardens 

 into cocoons. The middle line of the back is marked by a special red- 

 ness 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 



FIG. 60. Anterior region of Earthworm. (After HERING.) 



Note the eight setae (s) on each segment. 



R-S f -, Spots between 9-10, IO-TI, indicate openings of recep^acula 

 seminis ; Ovd., openings of oviducts on segment 14 v.ct., openings 

 of yasa deferentia on segment 15. 



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 is usually necessary. Almost always plain on the I5th ring 

 are the two swollen lips of the male ducts, less distinct on the 1 4th are 

 the apertures of the oviducts through which the eggs pass, while on 

 each side, between segments 9' and 10, 10 and n, are the openings 

 of two receptacula seminis or spermathecce into which male elements 

 from another earthworm 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. 



