564 



Seminal 

 vesicles 



Oviduct 



EVOLUTION OF ANIMALS 



Nerve cord 



Part V 



Lateral nerves 



Seminal 

 receptacles 



Vas deferens 

 Testis 



Sperm funnel 

 Vas deferens 



Ovary 

 Egg funnel 



Opening, 

 oviduct 



Opening 

 sperm duct 



Fig. 28.11. Earthworm. The hermaphrodite reproductive system composed of 

 complete male and female organs. (After Vogt and Yung. Courtesy, Brown: 

 Selected Invertebrate Types. New York, John Wiley and Sons, 1950.) 



are ciliated and constantly moving; in another they are seedlike spores packed 

 in a capsule. 



The mating of earthworms is a complicated process, not simply the shed- 

 ding of the sex cells into the water as in the aquatic annelids. Although earth- 

 worms are hermaphrodites they usually extend the forepart of their bodies 

 and mate with worms of nearby burrows (Fig. 28.4). The heads of the worms 

 are pointed in opposite directions with the ventral sides in contact. The clitel- 

 lum of one worm is opposite to segments 9 to 1 1 of the other worm. Mucus is 

 secreted until each worm is cloaked in a mucous tube that extends from seg- 

 ment 9 to the hind edge of the clitellum. As the sperm cells are discharged 

 from the openings of the sperm ducts on segment 15, they are carried back- 

 ward through two grooves to the sperm receptacles of the partner. This ends 

 the mating process and the worms separate. The clitellum secretes a mucous 

 belt which is shifted forward, along with the mucous tube, and finally over the 

 head of one worm. As this elastic belt passes the openings of the oviducts, the 

 mature eggs are evidently expelled into it. Farther forward, on segments 9 and 

 10, it apparently receives the sperm cells deposited there by the partner worm 

 during mating. The sperm and eggs join and fertilization is completed within 

 the mucous belt that in the meantime is slipping forward and finally off the 



