86 LIFE-HISTORY OF PARASITES. 



and "wrong" hosts, there are also hosts which only partially satisfy 

 the needs of parasites. In such hosts the invading parasites do not 

 perish immediately after their introduction; on the contrary, they 

 begin to develop as usual, and continue to do so up to a definite point, 

 without, however, completing their development. Whether the worms 

 continue for a longer time in this immature condition, or whether 

 they perish earlier, will apparently depend upon the degree to which 

 the conditions of development influence the life of the parasite. These 

 may vary in individual cases. 



Moreover, the above-mentioned phenomenon may be confirmed by 

 other examples. If a rabbit be fed with the bladder-worms of the 

 common tape- worm of the dog, they not only undergo the usual changes 

 during their sojourn in the stomach (as has been already observed above 

 (p. 75) with regard to the bladder-worms of the pig originating from 

 Tccnia solium), but they also delay for some time in the small in- 

 testine, fastening themselves to the intestinal wall just as in ordinary 

 cases. Some of them even develop a short segmented chain, which 

 differs from the normal beginning of the body of the tape-worm only 

 in that its segmentation is less complete. But here the development 

 stops, until after about ten to twelve days the young worms have all 

 perished (von Siebold, Kiichenmeister). A similar result is obtained 

 by feeding with Tccnia ccenurus. Since the bladder- worm of this tape- 

 worm usually develops only in the brain of lambs, one would expect 

 that the embryo would migrate only thither. But this is not the case, 

 as has been already mentioned (p. 67). The embryos scatter hither 

 and thither from the alimentary apparatus into the diverse organs of 

 the host ; but everywhere, with the exception of the brain, they perish 

 very soon after their entrance. If the infected lamb be examined 

 about three weeks after the feeding, one finds, besides a number of 

 small rounded bladders situated in the brain, which are the first 

 beginnings of the future Ccenurus, also numerous white knots, having 

 the appearance of tubercles, which are situated in the liver and 

 lungs, and more especially between the muscles, and upon closer exa- 

 mination can be recognised as cysts, which have developed round the 

 embryos. Sometimes these cysts may Contain the young of the invad- 

 ing tape-worm small, more or less opaque, and shrunken bladders 

 more rarely they are unchanged and alive, like those in the brain, 

 although for the most part less in size. x In rare cases one of these 

 small bladders may develop in time into a complete Coenurus. 2 



1 See on this head, and on the development of Coenurus in general, Haubner, GurU's 

 llfagazin filr pathol Anat., pp. 248 and 375, 1854. 



3 Thus Eichler found a developed Coenurus under the skin of the sheep, Nathusius 

 under the skin of an ox. Also in the rabbit the Coenurus has been observed in the 

 peripheral organs, but never as yet in the brain. 



