THE TREMATOD A. 93 



quaiiited, hoAvever, by the researches of Baer with a 

 remarkable entozooii of the Distoma family, Asjpidog aster 

 concldcola which occm^s only in the ventricle of these 

 molluscs. Upon comparing the form of this worm with 

 that of the body of the distoma-larva with clavate tail, I 

 cannot help supposing, from what I know of the change 

 consequent upon metamorphosis in this family, that the 

 B. ch(plicatum of Baer is the larva of Asjpidogaster con- 

 chicola (Baer.) There is certainly a remarkable discre- 

 pancy between this view and the fact that Baer has found 

 only one or extremely few specimens of Aspidogaster in 

 each muscle, whilst on the contrary the animals from 

 which I suppose them to proceed, occur by hundreds and 

 thousands in each individual ; this only proves, however, 

 that certain favorable conditions are pre-requisite for the 

 complete development of the entozoon, and how much 

 nature compensates by an inexhaustible fertihty, for the 

 inevitable loss sustained in consequence of the absence 

 of such favorable conditions. This discrepancy, more- 

 over, is not greater than that which exists in the case of 

 the Cercaria echinata, of which many thousands have 

 been observed, while it seems that the full-grown Bisto- 

 mata have been noticed only in the few individuals men- 

 tioned above. 



\¥hether my supposition, that the Cercaria {Distoma 

 duplicatum Baer) is changed into Aspidogaster conchicola 

 (Baer) be correct or not, still it is certainly a larval form, 

 and derives its origin in the third remove from a ciliated, 

 free-swimming animalcule, which is afterwards metamor- 

 phosed into a sacciform "nursing animal," which, in 

 accordance Avith the terms used before, must be called 

 a "■ j)arent-nurse' {grossamme) ; whilst the progeny deve- 

 loped in it, which, from their absolute and relative size, and 

 their possessing a cavity containing germs, can in no wise 

 be CercariöB, constitute evidently the same step in develop- 

 ment, as the true " nurses,'' and from which is produced 

 the next step in the series, the I), duplicatum, which can 

 only be compared with a true Cercaria; which in fact it is. 



