34 THEORY OF THE ORIGIN OF PARASITES. 



parasites. Steenstrup was not content with solving the enigma merely 



by hypothesis ; he also endeavoured by direct observation to place his 



Fig, 21. opinions beyond any doubt. He discovered 



that these Cercariae (Figs. 21 and 22) frer 



quently made their way into the body of 



Fig. 22. water-snails by boring through the muscular 



wall, and that, after losing their tails, they 



became encysted, resembling closely small and 



as yet asexual Trematodes. These facts were 



certainly not absolutely new, but those few 



naturalists who had anticipated Steenstrup 



in the discovery of the encysted condition 



of Cercariee, erroneously formed the opinion 



Figs. 21 and 22.-A free ^^^^ ^^^^ process, instead of being the pre- 

 and an encapsuled Cercaria, cursor of a further development, led merely 

 the latter without tail. ^^ ^^^ ^^^^-^ ^j ^-^^ parasite. Moreover, Steen- 



strup himself fell into an error when he supposed that the tailless Cer- 

 caria arrived at complete maturity within the body of the original 

 host; von Siebold,^ who shortly after adopted the opinions of the 

 illustrious Dane, rightly compared the development of the Cercaria 

 to that of Boihriocephcdus (Schistocephalus) solidus and Ligula, and 

 believed that its further growth would not take place until the 

 original host was devoured by some other animal. 



The older investigators (von Baer, see p. 30) had already demon- 

 strated the origin of the Cercarise ; but Steenstrup went further than 

 his predecessors in showing the identity of the " kingsyellow worm " 

 and the "living matrix of the Cercarise" with the "necessary 

 parasite" within the body of the embryonic Monostomum, though 

 the resemblance had been previously pointed out by von Siebold. 

 According to Steenstrup, the egg of the Trematode, expelled 

 from the body of its host, gave rise to a free larva, which 

 after a period of independent existence changed again into a parasite 

 (the "generative sac") after casting its skin. This parasite, how- 

 ever, did not at once become a Distomum, but stiU remained a 

 larval form (the asexual generation or so-called " nurse "), and in it 

 was subsequently developed, asexually from germ-granules, another 

 active larval form, the Cercaria from which the sexual adult then 

 took its rise. 



If it had been known before that the life-history of an animal 

 could be divided into several cycles, this process of development 

 would have been thoroughly understood some years earlier. The 



^ " Berioht uber die Leistungen, &c.," Archivf. Naturgesch., Jahrg. xiv., Bd. ii. p. 

 321, 1848. 



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