172 



COMPARATIVE ANATOMY 



CHAP. 



Fig. 120.— Oystioerous oellu- 

 lossB. Finn of T^nia solium, 

 cut in half. The scolex, which 

 is invaginated into the vesicle, is 

 seen with its suckers and rostel- 

 lum. After Leuckart. 



one of gemmation, we hold it to be simply one of growth and 

 differentiation. The head with the vesicle is, according to our opinion, 

 a young sexless Cestode answering to the scolex of Tcenia cucumerina in 

 the body cavity of the louse, only in this case the trunk or proscolex 

 becomes extended into a large vesicle by the accumulation of fluid 

 before the head of the tapeworm with its suckers, etc., forms. The 

 development of this vesicle ought to be 

 regarded as a special adaptation for the pro- 

 tection of the head. If such a Cijsticercus 

 reaches, ivith the tissue of its host, the 

 intestine of a new host, not only the cyst, 

 but the whole vesicle dissolves, while the 

 head and rudimentary neck which are 

 evaginated resist digestion because of the 

 calcareous bodies they contain. In other 

 words, the young, sexless, unsegmented tape- 

 worm loses its vesicular trunk. The scolex 

 fastens itself, by means of its organs of 

 adhesion, to the intestinal wall, and at once 

 regenerates the lost portion of the body in 

 the form of the first proglottis, which in the developed tapeworm 

 chain at length becomes the last and oldest, and new segments 

 follow this one. 



The vesicle of the Cystkercus of different tapeworms varies in size 

 according to the amount of fluid contained. It is sometimes a large 

 sphere, sometimes merely a small swelling at the posterior end of 

 the worm-like Cystkercus. 



In a few tapeworms development is complicated by the occurrence 

 of an alternation of generations, the young unsegmented form in the 

 intermediate host, the finn, multiplying asexually by gemmation. On 

 the wall of the finn there thus arise, not only one rudimentary head, 

 but several, indeed very many heads. Such a finn is called a Ccenupus. 

 It occurs in Tarda cmmirus. In the finn known as Echinococcus (of 

 Tcenm Echinococcus of the dog) there arise internally in the vesicular 

 body by invagination of the wall numerous daughter vesicles, and 

 even two generations of such vesicles, on whose walls several heads 

 form. 



We must further remark here that asexual scolices living free in 

 water have been observed. 



The Influence of Parasitism on the Structure and Development 



of Animals. 



In the race of the Platodes, for the first time among the Metazoa, the parasitic 

 manner of life is met with as a very widely spread phenomenon. Of the three classes 

 which form this race, the two classes of the Trematoda and the Cestoda consist 



