ORIGIN OF PARASITES 17 



tapeworm can be acquired, and also the rearing of cysticerci in 

 one's own body is prevented by avoiding the introduction of the eggs 

 of tapeworms. 



Though these results were definitely proved by numerous 

 researches, yet they have been repeatedly challenged, notably by 

 J.. Knoch (1862) who, on the 'basis of experiments, sought to 

 confirm a direct development without an intermediary host and 

 ciliated stage, at all events as regards Bothriocephalus latus. 

 However, the repeated communications of this author met with 

 but little favour from competent persons, partly because the 

 experiments were conducted very carelessly, and partly because 

 their repetition on dog and man (R. Leuckart) had no results 

 (1863). It was only in 1883 that I was able to prove that the 

 developmental cycle of Bothriocephalus latus is similar to that of 

 the other cestodes. The results obtained in other places by 

 Parona, Grassi, Ijima and Zschokke render any discussion of 

 Kuchenmeister's conclusions unnecessary. Long after Knoch a 

 French author, P. Megnin, also pleaded for the direct develop- 

 ment of some cestodes, and especially some taeniae ; he also sought 

 to prove a genetic connection between the hookless and armed 

 tapeworms of mammals (1879), but the arguments he adduced, 

 so far as they rest on observations, can be easily refuted or 

 attributed to misinterpretation. Only one of -these 'arguments is 

 correct, namely, that the number of the species of taeniae with 

 which we are acquainted is far larger than that of the corresponding 

 cystic forms ; but this disparity alone cannot be taken as a proof 

 of direct development. We can only say that our knowledge in this 

 respect is deficient ; as a matter of fact, we have during recent 

 years become acquainted with a large number of cystic forms 

 hitherto unknown appertaining to taeniae which have long been 

 known. It must also be borne in mind that no man in his lifetime 

 can complete an examination for bladder-worms of the large number 

 of insects, for instance, which may destroy an entire generation 

 of an insectivorous species of bird within a small district. 



Naturally it does not follow that direct development in the 

 cestodes is altogether lacking, but the arguments so far advanced 

 point only to its being possible. Indeed, communications from 

 Grassi (1889) have furnished us in Tcenia murina with an example 

 which shows that development may take place without an inter- 

 mediary host, notwithstanding the retention of the cystic stage. We 

 can no longer doubt that the oncospheres of this species, introduced 

 into rats of a certain age, after a time grow into tapeworms without 



