ORIGIN OF PARASITES 13 



the embryos, or oncospheres, of tapeworms furnished with six 

 hooklets in the so-called eggs of the Taenia;- while Creplin (1837) had 

 discovered the " infusorial" young of the Diphyllobothriuin (Bothrio- 

 cephalus) ditremum, and conjectured that similar embryos were to be 

 found in other cestodes with operculated eggs. At all events, the fact 

 was established that the progeny of the helminthes appeared in various 

 forms and was 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. I. Steenstrup (1842) was, however, the first to furnish explana- 

 tions for the numerous isolated and uncomprehended discoveries. 

 Commencing with the remarkable development of the Ccelenterata, 

 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 Distoma, etc., 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 

 the snails he had observed accompanied by the simultaneous loss of 

 the propelling tail, finally penetrated into other animals, in which they 

 became flukes. 



Part of this hypothetical cycle of development was erroneous, and 

 in other particulars positive observation was lacking, but the path 

 pursued was in the right direction. Immediately after the appear- 

 ance of Steenstrup's celebrated work, v. Siebold expressed his opinion 

 that the encapsuled flukes certainly had to travel, i.e., to be trans- 

 mitted with their bearers into other hosts, before becoming mature. 

 This view was experimentally confirmed by de Filippi, La Valette 

 St. George (1855), as well as by Pagenstecher (1857), while the meta- 

 morphosis of the ciliated embryo of Distoma into a germinal tube 

 was first seen by G. Wagener (1857) in Gorgodera (Distoma) cygnoides 

 of frogs. All that we have subsequently learned from the works of 

 numerous investigators about the development of endoparasitic trema- 

 todes has certainly increased our knowledge in various directions, and, 

 apart from the deviating development of the Holostomidce has, as a 

 whole, confirmed the briefly sketched cycle of development. 



Steenstrup's work on the cestodes did not attract the same atten- 

 tion as his work on trematodes. Steenstrup always insisted on the 

 "nurse" nature of the cysticerci and other bladder-worms. Abildgaard 

 (1790), as well as Creplin (1829 and 1839), had already furnished 

 the information that certain sexless cestodes (Schistocephalns and 



