96 DEVELOPMENT OF 



some oval, very active, ciliated animalcules, which were 

 not unlike the Opalina ranarum of Ehrenberg.* 



The Bistomata thus undergo their complete metamor- 

 phosis within these last-mentioned " nursing animals'' as 

 is the case with that Distoma which, as we have seen, 

 results from the larva of Cercaria armata when it has not 

 had the opportunity of quitting the " nurse' at the proper 

 time. Consequently, when the larvae of this species are 

 not destined to swim about freely, we cannot expect that 

 they should have the form of a Cercaria, that is, that 

 they should be provided with tlie tail, which is the prin- 

 cipal swimming organ of those creatures, and thus the 

 larvse of the different species of Bistomata which inhabit 

 the interior of the alhed fresh-water snails, would even 

 not have that form ; still less should we expect that the 

 Bistomata of other classes of animals should have it, and 

 we may even with greater probability suppose, that the 

 cercaria-iotYVß. will be found only in the Bistomata of the 

 snails, since, at present, the Cercarice have been observed 

 only in water, in which fresh-water snails have been.f 



I must here, however, refer to an observation which 

 although it proves nothing respecting the alternate gene- 

 ration of the Trematoda, yet makes us at least acquainted 

 with an interesting metamorphosis which some of the 

 forms undergo, and shows us where, in all probability, 

 the larvcB and pupa of a whole genus of Trematoda are to 

 be sought for. Nordmann's discovery of the occurrence 

 of entozoa, and especially of those belonging to the tre- 

 matode family in the eyes of fish, particularly of fresh- 

 water fish, in such numbers that their presence in these 

 delicate organs may almost be considered as normal, 

 caused at first, as might be expected, great astonishment, 



* An animal which swarms in the rectum of the frog in thousands, pro- 

 bably to the end that one of the many may reach a situation favorable to 

 the development and propagation of the species. Baer's Bucephalus poly- 

 morphus is probably also to be here considered a " nursing" animal. 



f A proof that entozoa may enter animals wliich they do not usually infest, 

 is derived from this, — ^that I have once found the pupa of Cercaria ecUnata 

 on the heart of an Anadonta. 



