22 ANIMAL PARASITES. 



fact from the Teenies, against which, as an unnatural separation, 

 Nitzsch (' Ersch and Gruber's Encyclop./ art. Anthocephalug), 

 P. S. Leuckart, and F. M tiller ('Archiv/ 1836, p. cviii) contended 

 in vain, in opposition to the great authority of Rudolphi. This 

 long interval only furnished the knowledge that even in cestode 

 worms, which were generically distinct from the Teenies, states 

 resembling vesicular worms occurred, namely, the vesicular genus 

 Anthocephalus, Hud. (= F/oriccps, Cuv.), which, as was subse- 

 quently ascertained, belonged to the Tetrarhynchi, and the Cysti- 

 cercus Lucii, Zeder, which the latter states he has found with the 

 remains of the caudal vesicle, and which was subsequently found 

 to belong to Tricuspidaria nodosa (Rhytis tricuspidata, Zeder). 



At last, in 1812, Steenstrup's theory of the Alternation of 

 Generations made its way here also. Steenstrup conjectured 

 that the cystic worms were early steps in the development or 

 generations of Helmintha which were unknown to him (Redi had 

 erroneously supposed that they belonged to Distoma of the 

 liver, and Tyson had also raised the same supposition, but only 

 to contradict it), and that they must be banished from the 

 system as a peculiar group, just as much as the asexual Trema- 

 toda (e. y., Cercaria, Leucochloridion, &c.) It is the more 

 remarkable that the Helmintha appertaining to the cystic worms 

 should have remained unknown to this distinguished zoologist, as 

 from the statements of Pallas and Goeze the relationship of 

 Teenia crassicoUis to Cysiicercus fasciolaris was easy to be seen, 

 and Nitzsch, F. 'S. Leuckart, and F. Mtiller had already re- 

 commended the abandonment of the separation of the cystic 

 worms from the tape-worms, because it was unnatural. It 

 appears to have been a peculiar fate which prevented the earlier 

 solution of this zoological problem ; naturalists either could not 

 correctly comprehend the true direction of progress, or, for 

 reasons which it is difficult to perceive, they ignored the labours 

 of their predecessors, which perhaps can only be realised when 

 we consider how universally the recognition of the truth takes 

 place slowly, and before it " a thousand years are as one day." 



In 1845, in consequence of Steenstrup's discovery, Dujardiu 

 (' Hist. Nat. des Helm./ pp. 544, 633) first asserted that the 

 cystic worms were undeveloped animal-forms and young states of 

 tape- worms ; and, indeed, that they were produced from those 

 germs of tape-worms, which, instead of the intestine, had got 

 into the parenchyma of the body of their host, and under the 



