THE TREMATOD A. 95 



which might with some probabihty be supposed to play 

 that part ; I must, however, remark upon this, that no 

 search has as yet been made for them with this view. 

 Secondly, that they may occur in such situations and 

 under such forms as would be least imagined ; and, 

 lastly, that they may have remained concealed by their 

 extreme minuteness. It should be remembered that the 

 " nurse' of the Medusa, Cyanea capillata is only a few 

 lines long, and remains at the bottom of the sea, while 

 the full-grown C. capillata which has been indebted to it 

 for its nourishment, is a million times larger, often spans 

 a space of several fathoms, with its tentacula, and swims 

 about freely in the ocean. 



There are not wanting, however, farther examples of 

 the occurrence of the same mode of development in other 

 Trematoda, besides those related in the above observa- 

 tions. In Baer's memoir, so often quoted, we meet vnth 

 mention of several, sometimes lifeless, (that is, motion- 

 less,) sometimes living, '■'germ-sacs' containing Cercarics 

 in aU stages of development, and which, consequently, I 

 am obliged to look upon as so many species of " nurses J" 

 Here belong also the cysts or so-called hydatids in which 

 Baer found completely developed Distomata ; as for in- 

 stance, D. cirrigerum in hydatids in the muscular sub- 

 stance of our common river crab {Astacus ßuviatilis,) 

 another Distoma in our smallest fresh- water snail {Ancy- 

 lus lacustiis,) and lastly, one which inhabits an active 

 "germ-sac," resembling the " nurses' of C. echinata, but 

 like those of C. ephemera, wanting the lateral processes. 

 The extraordinary entozoon Leucochloridium paradoxum 

 found by Carus in the swoUen tactile horns of Succinea 

 amphibia, and in which there were numerous developed 

 Distomata, is also only a " nurse." I cannot therefore 

 agree in the opinion of that ingenious naturalist, that it 

 might have originated by equivocal generation from the 

 metamorphosed cellular tissue of the mollusc ; I even 

 believe that I have discovered its true origin, in finding in 

 the tentacles of this snail in the early months of summer, 



