208 GENERAL ZOOLOGY 



of the male reproductive cells are also set free within the 

 moving band. When it pasess off the body, the ends con- 

 tract and a little hollow cocoon is formed. The eggs are 

 fertilized inside the cocoon and undergo the usual series of 

 developmental stages (Fig. 64). The first young worm to 

 hatch usually devours his younger relatives, so that as a 

 rule only one worm is produced from each cocoon. A " fer- 

 tilized " earthworm does not use all its stored sperm at once, 

 but may produce a dozen or more cocoons after each mating. 



GENERAL REMARKS ON EARTHWORMS AND OTHER 

 CH^ETOPODS 



An earthworm's body is made up of a series of very simi- 

 lar segments and more than half of these may be lost without 

 destroying the ultimate effectiveness of the bodily machine. 

 If the control of the earthworm's functions was strongly 

 ocalized at the anterior end, as in some bilaterally symmet- 

 rical animals, such a loss could not be sustained. In this 

 particular, then, the worm presents a primitive condition of 

 metamerism, which is said to be homonomous, because the 

 body segments are much alike. 



An earthworm's nervous system is not strongly central- 

 ized but is made up of a series of short overlapping units. 

 It consists of a dorsal ganglion near the anterior end, two 

 nerve trunks around the esophagus, and a ventral chain of 

 metameric ganglia throughout the rest of the body. As a 

 worm crawls along the impulse for contraction originates at 

 one end and is passed slowly down the whole length. For 

 example, if the circular muscles at the anterior end contract, 

 the same set of muscles is progressively stimulated, and a 

 wave of contraction passes backward. If the worm is cut 

 in two and then stitched together with a thread so that 

 there is a space of several inches between the cut surfaces, 

 the worm moves about as if it had a continuous body; the 

 pull exerted by the forward piece stimulates the one behind 

 to move in the usual way. Such behavior shows that the 



