166 Dr. E. Korschelt on the 
Since this larval organ has not only been proved to exist in 
the free-swimming Trochophora of Teredo (Hatschek), but 
was also described for the very degenerate Yrochophora of 
Cyclas (Ziegler), it is hardly open to doubt that it is present 
in Dretssena also; the more so since the agreement of the 
larve of Dreissena with those of the marine Lamellibranchs 
is in other respects complete—a fact to which I would call 
attention once more. ‘his appears in a peculiarly striking 
fashion on instituting a comparison with the figures which 
Lovén (loc. cit.) gives of various larve (belonging to Car- 
dium, Modiolaria, Montacuta, and other genera which are not 
further specified). The larva in its youngest stages provided 
with a bivalve shell exhibits a very remarkable agreement 
with that of the European oyster, as figured by Huxley *, 
Mobius f, and Horst (doc. cit.). The shape of the shell, too, 
at first resembles that of the shell of the young oyster-larva. 
It is almost circular, though it appears flattened on the dorsal 
surface, owing to two straight lines which there come into 
contact (fig. 1). It subsequently becomes more circular ; with 
further growth the shell becomes arched; this takes place 
especially in the region of the cardo, and leads to the forma- 
tion of the umbo. The shell is now not unlike that of a 
Jardium. It still consists solely of a membrane of conchiolin ; 
the secretion of the calcareous matter does not take place 
until later. 
The larvee rove about for some eight days, and during the 
greater portion of this period are found chiefly on the upper 
surface of the water. In Lake Tegel multitudes of pelagic 
alge were collected at the same time, and these doubtless 
provide the larve: with sufficient nourishment. When they 
are still thoroughly capable of active progression by aid of 
the velum they sink from the surface to the bottom of the 
water. This may either be explained as resulting from 
changes which now take place in their organization or may 
be traceable to the fact that food is perhaps easier of acqui- 
sition in the depths. 
While the larva was still enjoying a pelagic existence 
certain changes set in, of which the most important is the 
formation of the foot. The latter appears as a protuberance 
between the mouth and anus, and quickly assumes a conical 
shape. It then grows in length, and is forced to become bent 
* H. Huxley, “ Oysters and the Oyster-Question,” English Illustrated 
Magazine, 1885. 
Copied in Jackson’s paper ‘* Phylogeny of the Pelecypoda,” Mem. Bost. 
Soc. Nat. Hist. vol. iv. no. vili., 1890. 
+ K. Mobius, ‘ Die Auster und die Austernwirthschaft,’ Berlin, 1877. 
