EARTHWORMS, LEECHES, ETC. 



103 



They pass through the septum between the thirteenth and 

 fourteenth segments and open through the ventral wall of the 

 fourteenth segment. The sperm cells do not fertilize the ova 

 from the same individual. When the reproductive elements 

 are ripe, two worms mate and there is a transference of 

 spermatozoa from each individual to the other. The clitellum 

 then becomes very much swollen, and finally forms a collar- 

 like structure about the body of the worm. As this slips 

 forward the ova are discharged into it, and a little further 

 forward the sperm cells that have been received during 

 copulation are also emptied into it. As it passes on over the 



FIG. 39. A group of marine worms. At the left a gephyrean, Dendro- 

 stomum cronjhclmi, the upper right-hand one a nereid, Nereis sp.,_the lower 

 right-hand one, Polynos bremsetosa. (From living specimens in a tide- 

 pool in the Bay of Monterey, California.) 



head both ends of the collar become sealed and thus a capsule 

 containing the ova and spermatozoa is formed. This capsule 

 lies in the ground until the young are hatched. Only a part 

 of the eggs in each capsule develop, the rest being used for food 

 by the growing young. 



The Marine Worms. To the genus Nereis belong many of 

 the large marine worms that are found on almost all sea 

 beaches. The head is provided with two pairs of eyes and 

 with several tentacles which act as feelers. The sides of the 



