PARASITIC ADAPTATION. lOI 



of parasites that few of them, of course, can be reviewed 

 here. In general it may be said that their prodigious fecund- 

 ity and the great vital resistance with which most of them 

 are endowed, enables species to survive and perpetuate their 

 kind amid varied destroying influences which otherwise would 

 bring about their extermination. The Tcznia inhabiting the 

 intestines of man affords us another example of extreme 

 parasitism accompanied by this remarkable development of 

 the reproductive function. Here is a creature so altered to 

 its degenerate existence that it has become devoid of mouth 

 or intestine, the body consisting of a head from which are 

 given off segments which remain united until there is formed 

 a band-shaped colony of from 1200 to 1300, streaming back 

 from its attachment for a length of from fifteen to twenty 

 feet or more. After about the six hundredth, each segment is 

 a mature and complete sexual individual, which later, as it is 

 pushed on by new segments formed at the head, becomes 

 filled with fecundated eggs. By the successive detachment of 

 these "ripe" segments (proglottids) and their passage from 

 the body of the host, it has been estimated that the unarmed 

 Tcenia of man {Tcenia saginata) might throw off in a 5^ear as 

 many as 150,000,000 of eggs. These may escape from the 

 uterus of the proglottid, or one to several attached proglottids 

 containing the eggs may become scattered upon the ground. 

 In either case, through their contamination of fodder or 

 drinking water, they may, if so fortunate, reach the stomach 

 of cattle where the ^'g^ shells are digested away and the con- 

 tained embryos set free. At this stage (proscolex) the 

 embryo possesses six minute hooks, arranged in three pairs, 

 by which it bores its way through the intestinal wall and 

 wanders to a muscle, where it comes to rest ; or, if it finds 

 its way into a blood vessel, it may be carried with the blood 

 to any organ of the body. Finding its place of lodgment, it 

 loses its hooks, increases in size, and the head of the future 

 tapeworm is then developed in an invagination of the cyst 

 wall with which the embryo has become surrounded. During 



