14 THE ANIMAL PARASITES OF MAN 



Ligiila) from the abdomen of fishes only become mature after their 

 transference to the intestine of aquatic birds. These passive 

 migrations were confirmed in an entire series of other cestodes, 

 particularly by v. Siebold (1844, 1848, 1850) and E. J. van Beneden 

 (1849), n t by actual experiment, but by undoubted observation. 



It was correctly believed that the ova or oncospheres penetrate 

 into certain intermediate hosts, in which they develop into unseg- 

 mented larvae. Here they remain until, with their host, they are 

 swallowed by some predacious animal. They then reach the intestine, 

 being freed from the surrounding membranes through the process 

 of digestion, and settle themselves there to form the adult chain 

 of proglottides. Though some few scientists, such as P. ]. van 

 Beneden and Em. Blanchard, deduced from these observations that 

 the bladder-worms (Cysticerci), which had hitherto been regarded as 

 a separate class of helminthes, were only larval Taeniae, this correct 

 view was not at first universally accepted. The foundation was too 

 slight, and van Beneden was of opinion that the Cysticerci were not 

 necessary, but only appeared incidentally. 



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 (1850) he conjectured that the 

 Taeniae underwent a deviating cycle of development. He \vas 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 change of host 

 such as occurred in other cestodes was not supposed to take place 

 (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 taeniae. 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 transmission into the correct 

 host, as, for instance, in the Cysticercus fasciolaris of mice, the host of 

 which is the domestic cat, and in which there is a seemingly normally 



