in PLATODES LIFE-HISTORY OF CESTODA 171 



regular manner in the endoparasitic Trematoda. The generation which 

 multiplies by fertilised eggs always reaches the full degree of organisa- 

 tion of the Trematoda:, the following generation, which reproduces 

 itself parthenogenetically and lives in other hosts, never reaches that 

 degree of organisation ; they are ripe extraordinarily early, and perish 

 after they have produced another generation, which also remains at an 

 early embryonic stage. The different generations are known as 

 Sporocysts, Redice, and Distoma generations. The regular alternation 

 of such generations is called Heterogeny. 



XVI. The Life-history of the Cestoda. 



From the fertilised eggs of the Cestoda there proceed, generally 

 while they still lie in their egg shells in the uterus, embryos which, 

 since they are provided with 6 hooks, are called the 6 -hooked embryos. 

 The fate of this embryo, which only in Bothriocephalus is ciliated and 

 swims about freely in water, is very different in different Cestoda. 

 In Tcenia cucumerina, which is parasitic in the intestine of the dog, 

 it enters the body of the dog louse, Trichodectes canis. It here gets 

 rid of the egg shell and reaches the body cavity, where it develops 

 into a small worm, at one end of which the head develops with 

 its rostellum and its 4 suckers, while at the other the pore of the 

 excretory system can be made out. The head is somewhat sunk into 

 the body. The body is filled with numerous calcareous granules. We 

 have here simply an unsegmented, not yet sexually developed, tape- 

 worm, which may be compared with a young Amphilina, or Caryo- 

 phyllceus, or Archigetes. Through the dog's habit of licking and cleaning 

 itself, the present host of this young form, which we may simply call 

 scolex, is liable to be swallowed. While the louse is digested, the 

 scolex withstands digestion, the calcareous granules neutralising the 

 acid juices of the stomach. It fastens itself to the intestinal wall, 

 and begins to produce, by terminal budding or strobilation, the chain 

 of proglottides in which the genital organs develop. 



In this simple case we have one and the same individual, from 

 the egg to the strobilising intestinal scolex. The 6-hooked embryo, 

 the scolex in the body cavity of the louse, and the strobilising 

 scolex in the intestine of the dog, are the same individual in various 

 stages of development and in various habitats. In most of the 

 Cestoda, in consequence of peculiar complications in the develop- 

 ment, this is by no means so clear. In a series of Cestoda, to 

 which Tcenia solium and T. saginata belong, the 6-hooked embryo 

 in the tissues of its host changes, by the accumulation of fluid 

 internally, into a vesicle surrounded occasionally by a special capsule 

 or cyst formed out of those tissues. . From the wall of this vesicle, 

 which is called Finn or Cystieereus, there arises, at the base of 

 an invaginated hollow cone, a tapeworm head with suckers, rostellum, 

 etc. (Fig. 120). While most investigators regard this process as 



