OEIGIN OF INTESTINAL WOEMS. 17 



It was a long while after the origin of the Cercarice was 

 known, before any explanation offered itself as to how the 

 parasitic Cercaria-ssics in water-snails and mussels arose, and 

 as to what became of the Cercarice, which, when fully formed, 

 always seemed to desire to leave the bodies of the dissimilar 

 parents in which they had been developed ; penetrating the walls 

 of their sacs, and boring through the substance of the bodies of 

 the snails and mussels into the water, where they at first creep, 

 and at length paddle swiftly about by the help of their tails. 



With regard to the origin of the CerazHa-sacs, it cannot be 

 supposed that they proceed from Cercaria, since, in these last, no 

 organs of propagation are perceptible. In this perplexity the 

 doctrine of equivocal generation was again invoked, and it was 

 assumed that certain glandular caeca of the digestive or sexual 

 apparatus, in the snails and mussels in which Orcarm-sacs are 

 found, were converted into such sacs, and produced Cercariae by 

 equivocal generation. This was, of course, a mere assumption 

 based upon no direct observation. 



Now I was so fortunate as to make a discovery by which 

 much light has been thrown upon the obscure history of these 

 Cercarm and Cercaria-s&cs. 



It was in the year 1833, whilst Fi S- n - Fi S- 12 - 



fulfilling my duties as district 

 medical officer (kreisphysicus) at 

 Heilsberg, in East Prussia, that 

 I had occasion to examine a large 

 number of specimens of those 

 Trematoda known to Helmintho- 

 logists by the name of Monos- 

 tomum mutabile, which were very 

 commonly found in the geese of 

 that locality, in the cavities which 



lie underneath the eyeballs. I convinced myself that these para- 

 Fig. 11. An infusoroid embryo of Monostomum mutabile which has just left the 

 egg. (See rny essay on this subject in Wiegmann's ' Archiv.,' 1835, i, p. 69.) a. Sucker. 

 b. Double pigment-spot, c. Sporule-cyst. Fig. 12. a. The sporule cyst left free by the 

 death of the infusoroid embryo, b. The same viewed laterally. This body closely 

 resembles the sporule cysts of Cercaria armata. 



their manifold forms undescribed in this place, merely referring to the descriptions and 

 figures which Von Bar, in his masterly ' Beitrage zur Keuntniss der niederen Thiere' 

 (' Nova Acta,' vol. xiii, pars 2a, 1826,) and Steenstrup, in his ' Alternation of Genera- 

 tions,' have given. 



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