NUTRITIONAL RELATIONS OF HELMINTHS. . 85 



the immunity from their attacks enjoyed by older animals is not 

 perhaps so strikingly shown as in this case. 



Nothing is known of the reason of these phenomena. It is quite 

 uncertain whether it be the nutritive conditions that influence the 

 parasite, or perhaps the greater readiness with which the young tissues 

 yield a path for its wandering. It is not, however, age alone which 

 limits the conditions of this individual development, for we may ob- 

 serve, although rarely, even among old sheep, fresh cases of staggers ; 

 whereas, on the other hand, among lambs we may find single instances 

 of complete immunity from these parasites. Thus, for instance, 

 Baillet mentions the case of a lamb which was fed with mature seg- 

 ments of Tcenia ccenurus nineteen times in the course of about eight 

 weeks, and still no Ccenurus was developed. 1 The experimental 

 helminthologist has often, and even under similar conditions, to 

 register most unexpected results. Thus the author had fed a dog with 

 a multiple-headed Ccenurus, and in about ten days the intestine was 

 filled with more than a hundred completely developed tape- worms ; 

 while in another similar experiment, after three weeks there were 

 found in the intestine of the dog only heads of tape-worms, with 

 bands of segments an inch long attached to them in a few cases only ; 

 and in a third experiment a decidedly negative result was obtained. 

 These three cases are, of course, exceptions, for we may accept it as a 

 rule that the Ccenurus-heads develop completely within three weeks 

 into sexually mature tape-worms. Usually, however, even after this 

 time one may find single immature tape-worms, sometimes even here 

 and there an isolated head ; but these are irregularities which depend 

 rather upon the parasite than upon the host. Analogous cases are 

 also observed in other helminthological experiments. 



Just as old sheep cannot be infected by means of the embryos of 

 Tcenia ccenurus, so in a similar manner muscle- Trichinae develop only 

 rarely in dogs, even though the embryos are found to wander in 

 masses from the intestines into the body-cavity. Similarly Pagen- 

 stecher and I were unable to obtain muscle- Tricliince in birds, although 

 thousands of pregnant animals lived in the intestine. In pigeons 

 the introduced worms never reached sexual maturity. They grew 

 and became similar in appearance to mature animals, but the sexual 

 organs remained without germinal substance (Leuckart). 



From these instances, it is apparent that the conditions of develop- 

 ment of parasites are circumscribed that is to say, besides the " right " 



1 Ann. Sci. Nat., s^r. 4, t. x., p. 190, 1858. In a similar manner Fiedler ("Zur 

 Trichinenlehre," Dcutschcs Archiv fur Idinischc Medicin, Bd. i., p. 68, 1865) reports the 

 case of a man who ate a piece of raw meat, which was strongly infected with Trichinae^ 

 without suffering from it. 



