14 THE ANIMAL PARASITES OF MAN 



the snails he had observed accompanied by the simultaneous loss of 

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

 become distomes. 



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 encapsulated distomes certainly had to travel, i.e., to be 

 transmitted 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 metamorphosis of the ciliated embryo of distoma into a germinal 

 tube was first seen by G. Wagener (1857) in Distomum cygnoides of 

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

 numerous investigators about the development of endoparasitic 

 trematodes has certainly increased our knowledge in various direc- 

 tions, and, apart from the deviating development of the Holostomides 

 has, as a whole, confirmed the briefly sketched cycle of development. 



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

 attention 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 

 (Schistocephalus and Ligula) from the abdomen of fishes only 

 become mature after their transference into the intestine of 

 aquatic birds, and these passive migrations were confirmed in an 

 entire series of other cestodes, particularly by v. Siebold (1844, 

 1848, 1850) and E. J. van Beneden (1849), not by actual experi- 

 ment, but by undoubted observation. 



It was conjectured correctly that the ova or oncospheres pene- 

 trate into certain intermediate hosts, in which they develop into 

 unsegmented larvae. Here they remain until, with their host, they are 

 swallowed by some predacious animal ; they then attain the intestine 

 freed from the surrounding membranes through the process of diges- 

 tion, and settle themselves there to form the adult chain of proglot- 

 tides. Though some few scientists, such as P. J. van Beneden and 

 Em. Blanchard, deduced from these observations that the bladder- 

 worms (Cystici), which had hitherto been regarded as a separate 

 class of helminthes, were only larval taeniae, this correct view was 

 not at first universally accepted ; the foundation was too slight, 

 and v. Beneden was of opinion that the cysticerci were not 

 necessary, but only appeared incidentally. 



