IGO Dr. E. Korsdielfc on tie 



wliicli open into tlie Baltic. Finally, and probably in a 

 similar fashion, it al.so 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 Dreissena 

 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 Cardium, 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, Neriti'na 

 JluviatiUs, since the latter is found on the outer side of break- 

 waters, while Dreissena occurs only on the inside (E. von 

 ]\lartens). In a verbal communication Prof, von Martens is 

 decidedly of the opinion that Dreissena now really lives in 

 fresh water only. 



Dreissena polt/niorpha 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 larvae 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 Dreissena of 



* In the discussion on this point nttention was drawn by Prof. Xehiin;r 

 to the fact that Dreissena po/i/morj)/ia peviously occurred ii: Xorth 

 Germany, and has been found in tlie Diluvium of East and Wost Prus,>.ia, 

 as he int'ormod the ' GesoUschaft ' some years airo (Sitznnjrs-Ik'r. I8s;5, 

 p. 68, " Uebor das fossile ^"orl<ommen von Cerrus dama, C'i/j>ri/iits rarjtio, 

 und Dreissena jwli/inorjiha in Norddoutschland *'). It is supposed tliat 

 Dreissc7]a (probably owinj; to unfavmnable climatic cor.ditions) was driven 

 away, and subsequently immi<2Taled once more. 



