THE TREMATODA. 67 



of the spring of this year, always met with the pupae of 

 the species here described, but also, only in the auricle, 

 although they occurred frequently in considerable num- 

 bers. 



In some of tlie snails there were only from two to five 

 pupae ; in most of them from ten to twenty, and in several 

 even many more than thirty. Among the individuals ob- 

 served, the number of which far exceeded a hundred, only 

 about ten were met with, in the auricles of which, none of 

 these creatm-es were found ; but of Cercarice themselves 

 or their " nurses^' I did not, during these months, meet 

 with the least trace. I have convinced myself by exami- 

 nations repeated almost weekly, that pupae all retaining 

 the same condition, occupy the auricle even after the 

 first ten days of June ; if, however, the pupa cases are 

 opened dming this period, the inclosed animalcules appear 

 to be on the point of leaving them, for they are unusually 

 brisk in their movements, (much more lively than those 

 which I observed in the spring months, after they have 

 newly escaped from the pupae,) have assumed the appear- 

 ance Qi flukes, and at least some of them, even before 

 leaving the pupa case, have lost their circlet of spines. 



From this it is evident, that the Cercarice remain for a 

 long time in the pupa state, probably from two to nine 

 months, as might also have been conjectured from the 

 observations of Nitzsch upon Cercaria ephemera,^' accord- 

 ing to which it is not improbable, that when left wholly 

 to themselves and to natural circumstances, they pass from 

 the pupa cases into a new form, which differs in some 

 respects from that which I have previously described and 

 figm'ed from individuals which have been hatched as it 

 were by artificial warmtji from the pupae contained in 



* Nitzsch (Beitr. z. Infusorieiik., od. Naturbesclireibuiig. der Zerk. und 

 Bacill.,) says (s. 17) that his specimens of Cercaria ephemera lived three 

 months after the bodies had assumed the pearly aspect (that is, had become 

 pupse,) and regrets much that, as he expresses it, he was so fooHsh as to 

 throw them away, under the supposition that they would become nothing 

 more than they were, although their fresh condition would have induced the 

 suspicion that they retained the germ of a new life. 



