38 THEORY OF THE ORIGIN OF PARASITES. 



Beneden had investigated the Entozoa of various predatory fish, 

 especially the shark and ray, 1 and made observations at all stages. 

 He frequently found in the stomach of the shark the digested remains 

 of various osseous fish with Tetrarhynchus heads, some of which 

 were still encysted, some free or nearly so, while others had 

 already buried themselves in the intestine of their new host, and 

 budded out a longer or shorter chain of segments. Van Beneden's 

 researches were so extensive, and dealt with so many different forms, 

 that they fully justified the generalisation that the transference of 

 asexual Entozoa takes place by means of the food of their host, which 

 had been, up till the present time, only proved in the case of Liquid 

 and Schistocephalus. It is not, however, our purpose here to enter 

 particularly into van Beneden's statements as to the development of 

 Cestodes ; we shall recur to it in a future chapter, and content our- 

 selves for the present with mentioning that a bladder- worm, according 

 to this celebrated zoologist, is by no means a pathological condition, 

 but is closely allied in structure and development to the head of a 

 Tetrarhynchus. 



The correctness of this opinion was soon verified by a new ex- 

 periment, which showed that bladder-worms, as von Siebold had 

 previously stated was the case in certain forms, after losing the 

 bladder, become developed into true tape-worms in the intestine of a 

 suitable animal (Fig. 26). The history of helminthology does not, 

 perhaps, contain a single other fact that created such a marked sen- 

 sation. It was, however, not merely the proof that bladder-worms, 

 which had for so long a time formed an impregnable fortress for the 

 theory of spontaneous generation, were really the immature stage of 

 tape-worms, that excited so wide an interest, but it was also the cir- 

 cumstance that Kiichenmeister, 2 the discoverer of this fact, did not 

 discover it merely by chance, but by direct experiment, by the method 

 of feeding, which is so easy to control and repeat, and has furnished 

 the same results in other hands. 



The idea of using this method of proving the nature of bladder- 

 worms was suggested by previous discoveries, but it had, notwith- 

 standing, been made use of by no observer. I say no observer, for 

 the attempts of Klenke in this direction 3 have really not the slightest 

 claim to be mentioned. The method has only proved of value in 

 modern times. The meaning of helminthological experiment was 



1 " Les vers Cestoides": Bruxelles, 1850. (Preliminary account in the Comptcs 

 Rendus Acad. Bdg., 1849). 



2 "Ueber die Metamorphose der Finnen in Bandwiirmer," Prager VierteJjahrsschnft, 

 1852. 



3 "Ueber die Contagiositat der Eingeweidewurmer : " Jena, 1844. 



