Development of Dreissena polymorpha, Pallas. 165 
owing to the great degeneration which has taken place in the 
velum in this instance it is less distinctly visible. 
The larvee of Dretssena are very minute, and are exceeded 
in size by various pelagic Infusoria of Lake Tegel. Indeed 
at first sight they themselves produce the impression of an 
Infusorian provided with a strong adoral zone of cilia, or they 
might be mistaken for a Rotifer if the velum happens to be 
in active rotary motion. 
At the season at which the majority of Dreissena are 
engaged in reproduction, this year, therefore, towards the 
end of the month of June and the beginning of July, the 
larvee occur in large numbers on the surface of the water, so 
that with the help of the fine pelagic net it is then easy to 
procure material. 
The chief features in the larval organization are recognizable 
from fig. 1—the bivalve shell (figs. 2 and 3, s), the velum 
with its retractors, and a ciliation, also found in the larvee of 
other mussels, in the neighbourhood of the anus. As in the 
Trochophora, the oral opening les behind the powerful zone 
of cilia of the velum. In older larvee there appears behind 
the mouth a peculiar pigmentation, which often has a bilobed 
appearance (figs. 2-4, pz), and which I was at first inclined 
to regard as the earliest indication of the byssus-gland ; the 
position of the latter, however, would not well agree with 
this. A closer investigation of the subsequent stages will be 
necessary before we can decide whether a more important 
significance attaches to this formation. The fore-gut, which 
is marked off tolerably sharply from the mid-gut, leads into 
the wide stomach, upon which two cxcum-shaped evagina- 
tions doubtless represent the rudiment of the liver. At an 
early period the section of the intestine following the stomach 
forms a coil, which subsequently increases in extent when a 
lengthening of the intestine sets in. 
On the dorsal side the adductor muscle appears earl Vaio) 
I have not as yet been able to determine with certainty the 
nature of a thickening which is found between mouth and 
anus, yet Iam inclined to regard it as the rudiment of the 
pedal ganglion, owing to the great agreement between its 
position and that of the pedal ganglion described by Hatschek 
for the Trochophora of Teredo *., 
It would have been very desirable to settle the presence of 
the primitive kidney in the Zrochophora of Dreissena, yet in 
the short time which I was able to devote to the investigation 
of the younger larval stages I did not succeed in finding it. 
* B, Hatschek, “ Ueber Entwicklungsgeschichte yon Teredo,” Arbeiten 
Wien. Zool, Inst. 3 Bd., 1881, 
