Development of Dreissena polymorpha, Pallas. 159 
excretory organ which is so important for the Tvrochophora- 
larva (KX. Ziegler). With this we have the chief character- 
istics of the Trochophora, but this stage which repeats the 
Trochophora in a modified condition merely represents a 
transitory period of the embryonic development. In the case 
of the Unionide the typical form of the Lamellibranch larva 
receives even less expression. Here the velum is entirely 
wanting, and a tuft of cilia in the anal region is all we have 
to remind us of the free-roving larva*. The ova of the 
Unionide likewise develop in the gills, and the larve which 
are expelled from them, provided with shells and shell-hooks, 
have already reached a tolerably advanced stage of develop- 
ment—with the free-swimming larve of marine Lamelli- 
branchs they have nothing in common. As is well known, 
they attach themselves to fishes, upon which they live para- 
sitically fora time t. This mode of life at once bears witness 
to the extent of the adaptation which they have undergone, 
and which explains the striking transformation of their 
organization as also the various modifications of their early ° 
developmental stages f. 
Since the development of the freshwater mussels which 
have been mentioned exhibits so greatly modified conditions, 
it seemed interesting to ascertain how Drezssena behaves in 
this respect. Asa near ally of the common mussel Dreissena 
possesses more the character of a marine form, and we might 
therefore expect to meet with free-roving larve in its case, a 
most unusual phenomenon for freshwater mollusks. On the 
other hand, it appears to follow from the manner in which 
Dreissena is supposed to have entered our waters that it has 
long been accustomed to a freshwater existence, and thus 
perhaps an influence might also have been exerted upon the 
development. It appears that Dredssena has been derived 
from the region of the Kuxine. It probably penetrated into 
our waters by continually travelling further up stream from 
the wide mouth of the Volga§. At any rate it was trans- 
ported by connecting canals into those water-courses also 
* C. Schierholz, “ Ueber die Entwicklung der Unioniden,” Denkschrift. 
k. Akad. Wiss. zu Wien, Math.-naturw. Cl. 45 Bd., 1889. 
t M. Braun, “ Postembryonale Entwicklung von Anodonta,” Zool. Anz. 
1 Jahrg., 1878. 
Schierholz, op. cit. 
F, Schmidt, “‘ Beitrag zur Kenntniss der postembryonalen Entwicklung 
der Najaden,” Arch. f. Naturgesch. 51 Jahrg., 1885, 
$ A. Goette, ‘Bemerkungen ber die Embryonalentwicklung von 
Anodonta piscinalis,” Zeitschr. f. wiss. Zool. 52 Bd., 1891. 
§ E. von Martens, “ Kine eingewanderte Muschel,” Der Zoologische 
Garten, 6 Jahrg., 1865. 
