M. Vogt on some Inhabitants of the Freshwater Muscles. 453 
M. Baér, and after him M. Pfeiffer, have noticed an Acarus . 
which dwells in the palleal cavity of the Naiades. M. Baér called 
this Acarus Hydrachna concharum. M. Pfeiffer, who was not then 
acquainted with M. Baér’s investigations, gave it the name of 
Limnochares Anodonte. 
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 hatchmg 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 youl of hens’ eggs, of an oval form, and about 1 mil- 
limetre to 15 long. The embryos quit the eggs very early ; ‘he 
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, 
