PARASITISM 325 



is so long that a new cycle in a second host is required 

 to bring the parasite to maturity. Such conditions 

 attend the lives of the tape-worms. The eggs of these 

 parasites leave the body of the host with his excrement 

 and presumably scatter upon the soil. It has been 

 found by experiment that should any of these eggs be 

 swallowed by the same host or a host similar to that in 

 whose body they were produced, they sometimes at once 



FIQ. 117. Eggs of Tsenia Saginata. 



Fia. 119. Head of Tsenia saginata. FIG. 118. Mature segments of 



(Mosler and Peiper.) Tsenia saginata. 



develop into the adult organism, but this is by no means 

 the rule and the chances seem to be in favor of their 

 being swallowed by some other animal in whose body 

 they develop differently. Thus taking as an example 

 the most common tape-worm of man, Ta3nia saginata, 

 the beef-worm, it is found that its eggs, presumably 

 taken from the soil by grazing cattle, develop in them to 

 an embryo in no way similar either in appearance or 



