ORIGIN OF PARASITES 13 



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 cercaria creep 

 out of the " royal yellow worms," which occur in freshwater snails 

 (Linnceus paludina), and are probably generated in these worms. 



Oken, in whose journal, I sis, Bojanus published his disco very , 

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

 are the embryos of distomes." Soon after (1827), C. E. v. Baer was 

 able to confirm Bojanus' hypothesis that the cercaria as a " hetero- 

 genous brood " originated from spores in parasitic tubes in snails 

 (germinating tubes). Moreover, Mehlis (Isis, 1831, p. 190) not only 

 discovered the lids of the ova of distomes, but likewise saw an infu- 

 sorian-like embryo slip out of the eggs of Monostomum flavum and 

 Distomum hians. A few years later (1835) v. Siebold observed the 

 embryos (miracidia) of the Monostomum mutabile, 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 

 origination of the latter from the embryos of trematodes as at all 

 events possible. Meanwhile, v. Nordmann (Helsingfors) had in 1832 

 seen the miracidia of distomes provided with eyes swimming in 

 water ; v. Siebold (1835) had observed the embryos, or oncospheres, 

 of tape-worms furnished with six hooklets in the so-called eggs of 

 the taenia ; while Creplin (1837) had discovered the " infusorial " 

 young of the Bothriocephalus ditremus, and conjectured that similar 

 embryos were to be found in other cestodes with operculated eggs. 

 The fact was, at all events, established that the progeny of the hel- 

 minthes appears in various forms and is partly free-living. The 

 researches of Eschricht (1841) were likewise of influence, as they 

 elucidated the structure of the Bothriocephali, and proved that the 

 encysted and sexless helminthes were merely immature stages. 



J. J. Steenstrup (1842) was, however, the first to furnish explana- 

 tions for the numerous isolated and uncomprehended discoveries. 

 Commencing with the remarkable development of the Coelenterata, 

 he established the fact that the Helminthes, especially the endopara- 

 sitic trematodes, multiply by means of alternating and differently 

 formed generations. Just as the polyp originating from the egg 

 of a medusa represents a generation of medusae, so does the germinal 

 tube (royal yellow worm) originating from the ciliated embryo of a 

 distomum, &c., represent the cercaria ; these were consequently 

 regarded as the progeny of trematodes, and Steenstrup, guided by 

 his observations, conjectured that the cercaria, whose entrance into 



