170 PHYSIOLOGY 



large vesicles which surround the spermary or testis. 

 These have three large lobes on each side and open to the 

 outside on the fifteenth segment. 



The worm is hermaphroditic, that is, both eggs and 

 sperm are present in the same animal, but self-fertiliza- 

 tion is prevented by the arrangement of the organs and 

 by the fact that the eggs and sperm do not mature at 

 exactly the same time in each animal. Cross fertiliza- 

 tion takes place in the following way. In the breeding 

 season two animals come close together with the ventral 

 surfaces touching, the anterior end of one directed 

 toward the posterior end of the other. The ninth seg- 

 ment of one thus comes in contact with the fifteenth of 

 the other, bringing the opening of the male organs 

 directly opposite the opening of the seminal receptacles. 

 Sperm cells then pass from each worm into the seminal 

 receptacles of the other. The worms then separate. 

 When the eggs become ripe the ring, or clitellum, noticed 

 around the middle of the worm begins to pass forward. 

 When it reaches the fourteenth segment the eggs pass 

 into it. When it reaches the ninth and tenth, sperm 

 cells obtained from another worm pass into it. It con- 

 tinues to pass forward until it slips over the head. By 

 contraction the ends close; a capsule has then been 

 formed which contains sperm and eggs belonging to dif- 

 ferent individuals. They unite and cell division and 

 differentiation follow. 



Classification. — The Vermes, or worms, are a large 

 group of animals that differ from each other materially. 

 Their bodies are elongated and bilaterally symmetrical, 

 with a marked distinction between the dorsd and ventral 



