36 THEORY OF THE ORIGIN OF PARASITES. 



winter, and proved thai the young larvae hatched in the spring make 

 their way into the interior of young caterpillars, just out of the egg 

 an important addition to our knowledge of the life-history of Entozoa. 

 But before these observations had been brought to a close, von 

 Siebold had already attempted to fashion the theory of the origin of 

 the intestinal worms according to the newer views, which, as we have 

 already seen, were receiving more and more support from the progress 

 of discovery ; and, with this end in view, he published a complete 

 account of all the known facts relating to the development and gene- 

 ration of these animals. 1 This work, as might have been expected 

 from the wide knowledge of the author and the well-deserved reputa- 

 tion he enjoyed as a naturalist, made a great impression, and spread 

 abroad the conviction that the secrets of the phenomena of ento- 

 parasitism were to be sought for in the migrations and transference 

 of parasites, and were not explicable by any hypothesis of spon- 

 taneous generation. This work did not contain much that was abso- 

 lutely new in the department of helminthology ; for even the opinion 







FIG. 23. The common bladder-worm of the pig, with invaginated head (A), 

 and with extruded head (B). 



as to the tsenioid nature of bladder-worms (Fig. 23) had been some 

 time previously advanced by Dujardin, although it was treated of in 

 detail for the first time in the article by von Siebold, and based upon 

 the striking resemblance (already pointed out by Pallas and Goze) 

 between the head of the bladder-worm of the mouse and that of 

 Tccnia crassicollis of the cat. 2 



Concerning the development of the bladder-worms, von Siebold 

 had, however, peculiar views. He did not agree with Dujardin in 

 regarding them as larval stages or " nurses," but considered them to 

 be pathological formations caused by certain external circumstances, 



1 Art. "Parasiten" in Wagner's " Handworterbuch der Physiologic," Bd. ii., p. 640 : 

 Brunswick, 1843. 



" Hist. Nat. des Helminthes," pp. 544 and 632, 1845. 



