516 



EVOLUTION OF ANIMALS 



Part V 



vas deferen; 

 cirrus pouch- 



cirrus. 



genital 

 pore 



genital 

 atrium 



lateral 

 !rve cord 



IS efferens 



ieminal 

 iceptocle 



ovary 



■ odtype 



vitelline 

 duct 



vitelline 

 gland 



transverse 

 excretory canal 



Fig. 25.14. Mature proglottid (or segment) of Taenia pisiformis, a tape- 

 worm of dogs, showing the male and female reproductive systems. Male System. 

 The male cells are produced by many minute testes; they are carried by micro- 

 scopic tubes (vasa efferens) to a larger tube {vas deferens) and discharged 

 through the genital pore during the mating between proglottids of the same or 

 of different tapeworms. Female System. Great numbers of microscopic eggs are 

 produced in the ovaries. They are moved through the oviduct into the vagina 

 and are there fertilized by sperm cells received from the mating proglottid. The 

 eggs are then moved backward into a small structure {odtype) where they re- 

 ceive yolk from the vitelline gland. The fully formed eggs then pass forward 

 into the uterus that becomes so crowded with them that it finally fills the whole 

 proglottid, a bag of eggs ready to develop into young tapeworms. (After Good- 

 child. Courtesy, Brown: Selected Invertebrate Types. New York, John Wiley and 

 Sons, 1950.) 



its head, about the size of a pinhead, attached to the intestinal wall. Posterior 

 to the short neck is the chain of proglottids which make up the body, from six 

 to 25 feet long in mature worms with about 1000 proglottids. Each mature 

 proglottid contains 150 or more testes and at least one complex ovary. Fer- 

 tilized eggs burst from the proglottids either before or after the latter are cast 

 out of the intestine. They are protected by shells and on moist soil or vegetation 

 the embryos may remain alive for weeks. 



When swallowed by hogs or man, the embryos hatch soon after reaching 

 the intestine. The embryos soon pierce the intestinal wall, enter the blood 



