M. Vogt on some Inhabitants of the Freshwater Muscles. 453 



M. Baer, and after him M. Pfeiffer, have noticed an Acarus 

 which dwells in the palleal cavity of the Naiades. M. Baer called 

 this Acarus Hydrachna concharum. M. Pfeiffer, who was not then 

 acquainted with M. Baer's investigations, gave it the name of 

 Limnochares Anodontce. 



The eggs of this Acarus are arranged under the external lamella 

 of the branchial lobes ; they form granular masses of a whitish 

 colour, which are very easily discerned through the thin mem- 

 brane which covers them. It is sufficient to remove this mem- 

 brane or tear it with a needle to lay bare the eggs, which are just 

 large enough to be visible to the naked eye. The vitellus, com- 

 posed of fatty globules, gives a whitish colour to these eggs, the 

 envelope being perfectly transparent. I know no eggs of arti- 

 culated animals which so readily admit of microscopical obser- 

 vation. The envelope is of a consistence sufficient to protect the 

 embryo against a gentle pressure ; so that it is easy to move the 

 egg under the compressor in any desired direction without in- 

 juring the inclosed embryo; we may also, without much diffi- 

 culty, succeed in removing this envelope by cautious pressure, 

 and liberating the embryo without any disfiguration. The eggs 

 are so numerous that there is no need to be sparing of them. A 

 freshwater muscle is rarely opened in winter the gills of which 

 do not contain hundreds of eggs in different stages of develop- 

 ment, and it is always easy to compare the structure of the em- 

 bryos with that of the young or adult animals, because the latter 

 occur always in large number on the gills and on the internal 

 surface of the mantle. The embryo carries the vitellus a long 

 time after the hatching on the dorsal surface of the body ; it 

 comes out of the egg having only three pairs of legs, whereas the 

 adult has four. 



Lastly, I found, during the months of June and July, a great 

 number of young fishes lodged in the gills of the freshwater 

 muscles. The first time that I made this observation, I could 

 scarcely believe my eyes, and at first I thought it was the effect 

 of an extraordinary chance. But I was deceived ; in a hundred 

 freshwater muscles opened in the months mentioned, I found, at 

 least in sixty, small fishes all belonging to the same species, at 

 different degrees of development. I found as many as forty in 

 a single freshwater muscle, the gills of which were then consi- 

 derably enlarged. I rarely met with eggs ; they were yellow, 

 like the yolk of hens' eggs, of an oval form, and about 1 mil- 

 limetre to 1£ long. The embryos quit the eggs very early; lb ft 

 youngest that I have met with could not yet move, and were so 

 little advanced, that the black pigment of the eyes had scarcely 

 begun to be deposited. The largest fishes which I met with in 

 the gills were 10 millimetres long; they swam with vivacity, 



