M. Vogt on some Inhabitants of the Freshwater Muscles. 451 
of perfect individuals, frequently swarm in the organs of these 
animals. During my stay at Giessen I undertook a series of 
researches on the embryology of the various animals which in- 
habit the freshwater muscles ; these investigations were inter- 
rupted by the revolution of 1848. Having no opportunity of 
resuming them immediately, I consider it a duty to call the 
attention of naturalists to a field of investigation which promises 
a rich harvest. Embryologists especially will find in the fresh- 
water muscles matter to satisfy them ; for they present not only 
their own eggs and the larve of bivalves which hatch in their 
gills, but also eggs and embryos of Entozoa, of articulated and 
even vertebrated animals. 
The eggs of the freshwater muscles pass into the gills in the 
beginning of May. I have not been able to observe the passage 
itself, but I have notwithstanding traced the development of the 
ege in the first stages of the embryogenic process. I have seen 
the division of the vitellus in all its phases, up to the formation 
of a globular embryo, which still wanted a shell. By comparing 
the eggs concealed in different parts of the gills, I convinced 
myself that the eggs placed near the anus were more advanced 
in this process than those which were in the anterior portion 
of the gills; the latter therefore appear to be filled from be- 
fore backwards. The eggs, in the ovary, arrived at a certain 
stage of maturity, are always composed of a transparent envelope, 
and of a granular vitellus of a whitish, yellow or orange colour, 
in which is situated the Purkinjean vesicle. This vesicle is very 
large, entirely transparent, and always contains two small vesi- 
cles (germinative spots of Wagner), one of which sometimes pre- 
sents a granular appearance. It is a general law, as regards the 
eggs of the Unios and Anodonts, that these spots are to the num- 
ber of two in each egg. 
The ovary and testicle are the habitual seat of those larvee of 
Trematoda, which M. Baér has designated under the name of 
Bucephalus polymorphus. The figures which M. Baér has given 
of these singular animals are tolerably correct. They are formed 
of a distomoid body placed on two long rolled-up appendices 
which have a serpentine movement. These larve are developed 
in the long filiform intestines, which, under the microscope, ex- 
hibit now and then swellings, in which are lodged the Bucephali. 
The sexual organ affected by this dyscrasy resembles a mass of 
entangled white threads; I found one individual in about two 
hundred freshwater muscles, the ovary of which had the appear- 
ance of a fibrous schirrus macerated for some time. These threads 
are especially developed in January and February ; and it is also 
in these months that the development of the Bucephali may be 
easily observed. In the swellings of the mtestines, globules 
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