150 



Platyhelminths. 



whicli are separated from one another by constrictions ; the segments 

 nearest to the head are the youngest, those farthest off, the oldest and 

 largest ; new segments are constricted off from the hind part of the 

 head. When the development of the segments has gone so far that 

 they contain ripe eggs in large numbers, they separate from the 

 cbain ; in some cases, they break off at an earlier stage, and grow 

 independently; in others, they remain in connection. The number of 

 segments in a chain may increase from quite a few to many hundreds.* 

 The adult worm occurs exclusively in the gut of the Yerte- 

 brata, to the wall of which it attaches itself by the organs of adhesion 

 on the head. It feeds by absorbing the contents of the gut through 

 the skin. The Tapeworm does not, however, spend its whole life in 

 the same place, or in the same host. At a younger stage, as a 



Fig. 108. 



rig. 109 b. 



Fig. 110. 



Fig. 111. 



Fig. 108. Six-hooked larva of T. Bolinm. 



Fig. 109. Prosoolex of the same Taenia, with (a) inpnshed, (6) evaginated 

 ad. 

 Fig. 110. Ciliated larva of Bothriooephalus latus. 

 Fig. 111. Prosoolex of above. 

 All figures enlarged. — After Leuokart. 



proscolex, it lives in another host, and in a different part of the 

 body. It then often consists only of the head ; in other cases, of 

 the head and of a number of joints, which are, however, never 

 ripe. A second host now devours the first, and thus a transference 

 of the parasite occurs, and it attains sexual maturity. The tem- 

 porary host is usually infected by taking the eggs, (or the 

 segments containing eggs), which have passed from the permanent 

 host with the excreta, into its alimentary canal. Each egg contains, 

 one larva, a small round organism provided with six hooks, which. 



* According to the opinion put forward by many observers in recent times, the 

 Cestode chain is not a stock, but is a single individual, with a great number of 

 gonads. But this view is contradicted by the fact that in some Cestoda, even after 

 separation, the segments are capable of growth, become ripe, copiilate, and thereby 

 testify abundantly to their independent individuality. 



