90 DEVELOPMENT OF 



In order to place in the clearest possible light that 

 which, in the development of the Distoma, is effected 

 through several alternating generations, I will adduce as 

 an observation precisely to the same point, that a minute 

 oval animalcule which moves by means of cilia, and in 

 that respect resembles the offspring of the ova of the 

 Bistomata, becomes in the third generation a Distoma- 

 like animal. There is found for instance both in the in- 

 ternal organs, and in the external mucus of our common 

 fresh-water muscles {Anadontd) an extraordinary quan- 

 tity of animalculse, appearing to the naked eye like minute 

 oval points, transparent, with milk-white extremities and 

 having a peculiar rotatory motion. Under the micro- 

 scope they appear reniform, very flat, with opaque extre- 

 mities, and as it seems are covered with vibratile cilia, so 

 that they call to mind a ParamcBcium or Colpodium, to 

 one of which genera they might not improbably be re- 

 ferred, had we sufficient means of determining the point. 

 I cannot avoid supposing, that the figs. 7, 8, 9, tab. 73, 

 (Zool. Dan.,) refer to this animalcule ; and it is certain, 

 that it is these animals which Baer, in his memoir on 

 Distoma diiplicatum mentions, under the name of Para- 

 mcecium, and describes as a " chaotic swarm" in the inte- 

 rior of the Anadonta. 



These ^aram(Scium-Y\kQ animalculse, crowd about in the 

 aqueducts, both in the mantle and in the foot and in- 

 ternal parts of the muscle, (as for instance in the kidneys;) 

 many of them may be observed in the same situation, 

 and it is then readily perceived that they are not all 

 equally active; for some possess all their cilia, whilst 

 others have lost them, and in consequence remain quiet, 

 or are seen to be adherent, and their substance has become 

 parenchymatous. There are also met with, especially in 

 the muscles inhabited by these Paramcscia, other more 

 parenchjrmatous, and more or less regularly oval bodies, 

 which are slightly depressed and adherent. It is not 

 difficult, among the numbers which are usually present, 

 to show the complete change in every respect of these 



