OKIGIN FROM FKEE-LIVING ANIMALS. 25 



strikingly like the Entozoa, and sometimes were even actual Entozoa. 

 Of special import was the discovery of a tape-worm in fresh water by 

 Linne,! and subsequently by several other naturalists in different 

 places. We now know that this tape-worm (Bothriocephalus v. Bchisto- 

 cephalus solidus) inhabits originally the body-cavity of the stickleback, 

 which it abandons at a certain stage of its development, and passes 

 some time in the water, being finally swallowed by a water-fowl. ^ 

 Linne, however, did not know these facts, and regarded the worm 

 without hesitation as a young and incomplete specimen of the large 

 human tape-worm (Boihriocephalus latus), and believed, therefore, 

 that this worm was conveyed into the body from the exterior, where it 

 already existed fully formed, in water. Moreover, this assertion was 

 not confined to the tape-worm ; Linne believed that he had also dis- 

 covered the liver-fluke of the sheep and the Oxyuris of man leading 

 a free existence ; but there is no doubt that he mistook a Planarian 

 for the former and one of the free-living Anguillulidse for the latter. ^ 



However small the evidence was, it appeared sufiicient to esta- 

 blish this idea, which was believed in by many naturalists after 

 Linne, chiefly because the facts known at that time about the 

 Entozoa, as well as other parasites which we shall have to consider, 

 were extremely fragmentary. To illustrate the small degree in which 

 helminthology was known at that time, it may be mentioned that in 

 spite of the vast numbers of existing Entozoa, not more than a dozen 

 — and these almost entirely human parasites — had been described. 



Soon after this commenced a new era in helminthology. The 

 knowledge of intestinal worms, which was till then chiefly of medical 

 interest and cultivated by medical men, gradually began, under the 

 influence of the Linnaean school, to attract the attention of zoologists. 

 Men of high ability and wide knowledge, like Pallas, 0. E. Miiller, 

 and others, bestowed upon this science their special attention, and 

 increased our knowledge of parasites in all directions. But every 

 new parasite and new host rendered less probable the idea of Linn^ 

 that these animals lived sometimes freely and sometimes as parasites. 



The number of known Helminths soon became very considerable, 

 but all attempts to find them living in a free state were in vain, 



' " Amcenit. Acad.," t. ii., Erlangae, 1787, p. 93. 



^ Steenstrup, Overs. K. dansk. Videnskah. SelsTc. Forhandl, p. 166, 1857 : Zeitschr. d. 

 gesammt. Naturwiss., Bd. xiv., p. 475, 1859. In a similar manner Ligula frequently 

 leaves the body of the fish in which it is parasitic at a certain stage of its development, 

 and leads a free life, see Blooh, " Abhandl. von der Erzeugung der Eingeweidewurmer," 

 p. 2, 1782. 



'' " Systema Naturae," ed. x., t. i., p. 648. Fasciola Iiepatica, "habitat in aquia dul- 

 cibus ad radices lapidum, inque hepate pecorum." Ascaria vermicularis, "habitat in 

 paludibus, in radicibus plantarum putresoeutibus, in intestinis puerorum et equi." 



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