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 Cercaria} (Figs. 21 and 22) fre- 

 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 Cercariae, erroneously formed the opinion 



FIGS. 21 and 22.-A free that this P roc ess, instead of being the pre- 

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

 to the death of the 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, 1 who shortly after adopted the opinions of the 

 illustrious Dane, rightly compared the development of the Cercaria 

 to that of Bothrioceplialus (Schistocephalus) solidus and Liyula, 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 Cercariae ; but Steenstrup went further than 

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

 and the "living matrix of the Cercarios" 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 still 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 



1 "Bericht uber die Leistungen, &c.," Archivf. Naturgesch., Jahrg. xiv., Bd. ii., p. 

 321, 1848. 



