THE EARTH WORxM. 147 



muscles. Suppose the body is extended, as described 

 in the preceding paragraph. Now the longitudinal 

 muscles contract. The tendency is to shorten the dis- 

 tance between the head and the tail. The head tends 

 to be drawn backward, but by this movement the 

 backward pointing setae that are near the head are 

 driven into the ground, while those at the posterior 

 of the body move freely over the surface ; hence the 

 tail is drawn forward. The result of these muscu- 

 lar contractions, alternately performed, is to send the 

 body forward. If the setae, however, are directed 

 forward, as they may be, the result of the same con- 

 traction is to send the body backward. 



3. The cross section shows the intestine, the 

 dorsal blood vessel, and the ventral blood vessel ; also, 

 the ventral nerve cord, lying below the ventral blood 

 vessel, with nerves extending to the muscles of the 

 body wall. The nerve cord seems to be composed of 

 three cords, or elements. 



4. The cross section shows eight setae, arranged 

 in four pairs. Each pair has a muscle by means of 

 which it may be retracted or thrust out, and directed 

 forward or backward. 



5. One group of reproductive organs is seen. 

 Both male and female reproductive organs are found 

 in every earthworm. The male reproductive organs 

 occur in some worms in segments 9, 10, and 11 ; the 

 female organs (ovaries), in segments 13 and 14. The 

 position of these organs varies somewhat with the 

 different kinds of worms. More than 35 different 

 kinds of earthworms occur in the United States. 



The earthworm is hermaphroditic, although (as in 

 the case of red clover and of many other flowers) cross 

 fertilization is necessary. That is, the male principle 

 of one worm fertilizes the female principle, or egg, of 

 another worm. The eggs of the earthworm are laid in 

 capsules, each capsule containing six to eight eggs. 



