160 Dr. E. Korschelt on the 
which open into the Baltic. Finally, and probably in a 
similar fashion, it also reached our rivers, and there wandered 
up stream, until at last it attained its present wide distribu- 
tion *, 
It has been reported from the Caspian Sea that Drecssena 
sometimes still occurs among marine mussels, though even 
here it confines itself to water which is less rich in salt, and 
is only found in the estuary of the Volga, which is probably 
largely impregnated with fresh water. Other truly marine 
mussels, such as Curdiwm, have been found in its vicinity in 
a dead state. In the Baltic it lives only within the bays, 
and it is interesting to note that here it seems to endure the 
strong salt water even less than a freshwater snail, Nerdtina 
fluviatilis, since the latter is found on the outer side of break- 
waters, while Dretssena occurs only on the inside (KH. von 
Martens). In a verbal communication Prof. von Martens is 
decidedly of the opinion that Dredssena now really lives in 
fresh water only. 
Dreissena polymorpha is consequently to be regarded more 
as a freshwater mussel than as a marine form. We see that 
the question as to how it behaves with reference to its deve- 
lopment is thereby brought still more closely home to us, for 
it would always be possible that a modification had set in in 
the mode of development. It is true that the small changes 
in the organization of the mussel itself do not support this 
theory. I was therefore able to search for the larve of 
Dreissena with some prospect of success. Since the mussel 
is of frequent occurrence near Berlin, and is even found in 
abundance in Lake Tegel, a successful treatment of the 
problem was rendered the more easy. 
Although it appeared to me to be probable that reproduc- 
tion took place in the spring, I nevertheless examined a 
considerable number of the mollusk in August of last year, 
and found them only moderately provided with sexual 
products or almost entirely destitute of them, However, the 
very small size of the ripe ova appeared to me even at that 
time to confirm the supposition of a free development. 
This year from March onwards I watched the setting in of 
the breeding-season both in the case of the Dredssena of 
* In the discussion on this point attention was drawn by Prof. Nehring 
to the fact that Drevssena polymorpha previously occurred in North 
Germany, and has been found in the Diluvium of Kast and West Prussia, 
as he informed the ‘ Gesellschaft’ some years ago (Sitzungs-Ber. 1883, 
p- 68, “ Ueber das fossile Vorkommen von Cervus dama, Cyprinus carpio, 
und Dreissena polymorpha in Norddeutschland”). It is supposed that 
Dreissena (probably owing to unfavourable climatic conditions) was driven 
away, and subsequently immigrated once more. 
