12 THE ANIMAL PARASITES OF MAN 



many helminthes known that never developed sexual organs and 

 never produced eggs, and which therefore were referred to generatio 

 aquivoca. People were convinced that the intestinal mucous mem- 

 brane or an intestinal villus could transform itself into a worm, 

 either in a general morbid condition of the body, or in pathological 

 changes of a more local character. The appearance of helminthes 

 was even regarded as useful and as a means for the expulsion of 

 injurious matter. 



These views, firmly rooted and supported by such eminent 

 authorities as Rudolph! and Bremser, could not easily be overthrown. 

 First, a change took place in the knowledge of the trematodes. 

 In 1773, O. Fr. Miiller discovered Cercarice living free in water. 

 He regarded them as independent creatures and gave them the name 

 that is still used at the present time. Nitzsch, who also minutely 

 studied these organisms and who recognized the resemblance of 

 the anterior part of their bodies to a Fasciola, did not, however, arrive 

 at a correct conclusion. He regarded the combination rather as 

 that of a Fasciola with a Vibrio, for which he mistook the charac- 

 teristic tail of the cercaria. He also noticed the encystment (trans- 

 formation into the "pupa") on foreign bodies of many species of 

 these animals, but was of opinion that this process signified only the 

 termination of life. 



Considerable attention was attracted to the matter when Bojanus 

 first published a paper entitled "A Short Note on Cercaria and their 

 Place of Origin." He pointed out that the cercarire creep out of the 

 " royal yellow worms," which occur in freshwater snails (Limiuva, 

 Paludina), and are probably generated in these worms. 



Oken, in whose journal, Isis (1818, p. 729), Bojanus published his 

 discovery, remarks in an annotation, " One might lay a wager that 

 these Cercariae are the embryos of Distomes." Soon after (1827), 

 C. E. v. Baer was able to confirm Bojanus' hypothesis that the 

 cercariae as a " heterogeneous brood " originated from spores in para- 

 sitic tubes in snails (germinating tubes). Moreover, Mehlis (Isis, 1831, 

 p. 190) not only discovered the opercula of the ova of Distoma, but 

 likewise saw the infusorian-like embryo emerge from the eggs of 

 Typhlocceliim (Monostomum) flavnm and Cathcemasia (Distoma) hians. 

 A few years later (1835) v. Siebold observed the embryos (miracidia) of 

 the Cydoccelum (Monostomum) mntabile, and discovered in their interior 

 a cylindrical body that behaved like an independent being (" necessary 

 parasite"), and was so similar in appearance to the " royal yellow 

 worms " (Bojanus) that Siebold considered the origin of the latter from 

 the embryos of trematodes as, at all events, possible. Meanwhile, 

 v. Nordmann of Helsingfors had in 1832 seen the miracidia of flukes 

 provided with eyes swimming in water; v. Siebold (1835) had observed 



