234 THE TAPEWORMS 



for instance, the male reproductive system reaches maturity 

 when the proglottid has been pushed back to about the 200th 

 position, whereas the female system does not mature until it 

 has been pushed 200 or 300 proglottids farther back. Copulation 

 then takes place by the doubling back of the chain of proglottids 

 on itself, bringing the young mature male segments into contact 

 with the older mature female segments. 



After copulation, when the mature fertilized eggs begin to form, 

 great changes take place in the proglottid. The uterus begins 

 to enlarge and branch until it nearly fills the segment, crowding 

 aside and absorbing the other organs. Segments thus distended 

 with eggs are spoken of as " ripe " proglottids and are ready to 

 break loose from the chain to be voided with the faeces of the host. 

 Ripe proglottids of a number of species of tapeworms found in 

 man are shown in Fig. 84. 



Life History. The life histories of all tapeworms are much 

 alike. Usually before the ripe proglottids become detached and 



pass out of the host, the 

 eggs develop, inside their 

 tough shell, into little 

 round embryos with six 

 hooks (Fig. 85 A). In 

 order to continue their 



FIG. 85. A, egg of beef tapeworm. T. saginata; -, > , 



- note contained embryo and external filaments; development Such em- 

 B, freed six-hooked embryo. X 300. (After bryOS must be eaten by 



another species of animal 



which acts as an intermediate host. Most often the adult 

 form of the worm occurs in carnivorous animals, while the in- 

 termediate host in which the larva develops is a herbivorous 

 animal, but there are numerous exceptions to this. There is no 

 active search for a new host on the part of the tapeworm embryo 

 as there is by the embryos of flukes, but instead merely a 

 passive transfer. 



When eaten by a suitable intermediate host, the shell enclosing 

 the six-hooked embryo is dissolved off, and the embryo is re- 

 leased (Fig. 85B). It migrates into the organs and tissues of 

 the body, aided by the blood and lymph circulation of the host, 

 some species having preference for certain organs, others es- 

 tablishing themselves with equal readiness in any parts which 

 they happen to reach. 



