ORIGIN OF PARASITES 15 



v. Siebold was a strenuous opponent to this theory, notwith- 

 standing his experiences on the change of hosts of the Tetra- 

 rhynchus. Together with Dujardin he conjectured that the taeniae 

 underwent a deviating cycle of development (1850) ; he was of 

 opinion that the six-hooked oncospheres left the intestine, in 

 which the older generation lived, and were scattered about with 

 the faeces, and finally re-entered per os (i.e., with water and food) 

 a host similar to the one they had left, in the intestine of which 

 they were directly transformed into tapeworms. A changing of 

 host in the sense of the other cestodes was not supposed to occur 

 (the history of the cestodes was at this time not entirely estab- 

 lished). As the oncospheres of the taenia are enveloped in one 

 calcareous or several softer coverings which they cannot leave 

 actively, and as, in consequence of this condition, innumerable 

 oncospheres cannot, penetrate into an animal, and others cannot 

 reach the proper animal, v. Siebold conceded, at least for the 

 latter, the possibility of a further development ; but this was only 

 supposed to occur because they had either invaded wrong hosts, 

 or, having reached the right hosts, had penetrated organs unsuit- 

 able to their development^ and had thus gone astray in their 

 travels, and had become HYDROPICALLY DEGENERATED T^NI^E. 

 This was v. Siebold's explanation of bladder-worms. Naturally, 

 v. Siebold himself conjectured that a recovery of the diseased 

 tapeworm might occur, in a few exceptional cases, after trans- 

 mission into the correct host, as, for instance, in the Cysticercus fascio- 

 laris of mice, the host of which is the domestic cat, and in 

 which there is a seemingly normally developed piece of tape- 

 worm situated between the caudal vesicle and the cysticercus head. 



Guided by correct views, F. Kiichenmeister undertook in Zittau 

 the task of confirming the metamorphosis of Cysticercus pisiformis 

 of hares and rabbits into tapeworms in the intestine of the dog, 

 by means of feeding experiments. The first reports on the 

 subject, published in 1851, were not likely to meet with universal 

 approval, because Kuqhenmeister first diagnosed the actual tape- 

 worm he had been rearing as Tcenia crassiceps, .afterwards as 

 Tcenia serrata, and finally as Tcenia pisiformis, n. sp. However, 

 in any case Kiichenmeister, by means of the reintroduction of 

 experimental investigation, rendered a great service to helminthology. 



The publication of Kiichenmeister's works induced v. Siebold 

 to undertake similar experiments (1852 and 1853), which were 

 partly published by his disciple Lewald in 1852. But the posi- 

 tive results obtained hardly changed Siebold's opinion, for 



