Development q/*Dreissena polymorpha, Pallas. 1G5 



owing to the great degeneration which lias taken place in the 

 velum in this instance it is less distinctly visible. 



The larva? of" Dreissena are very minute, and arc exceeded 

 in size by various j)elagie Infusoria of Lake Tegel. Indeed 

 at first sight they themselves produce the impression of an 

 Infusorian ])rovidcd with a strong adoral zone of cilia, or they 

 might be mistaken for a Rotifer if tlie 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 

 larvoj 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 larvse of 

 other mussels, in the neighbourhood of the anus. As in the 

 Troclwphora, the oral opening lies behind the powerful zone 

 of cilia of the velum. In older larvaj there appears behind 

 the mouth a peculiar pigmentation, which often has a bilobed 

 appearance (figs. 2-4, ^;i), and which I was at first inclined 

 to regard as the earliest indication of the byssus-gland j 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 raid-gut, leads into 

 the wide stomach, upon which two cjecum-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 early (fig. 1) . 

 I have not as yet been able to determine with certainty the 

 nature of a thickening which is found between mouth and 

 anus, yet I am inclined to regard it as the rudiment of the 

 pedal ganglion, owing to tlie great agreement between its 

 position and that of the pedal ganglion described by Hatschek 

 for the TrocJiopJiora of Teredo *. 



It would have been very desirable to settle the presence of 

 the primitive kidney in the Trocliophora of Dreissena, yet in 

 the short time v.'hich I was able to devote to the investigation 

 of the younger larval stages I did not succeed in finding it. 



* B. Hatschek, " Ueber Entwicklungsgeschichte von Teredo'' Arbeiten 

 Wieu. Zool. Inst. 3 Bd., 1881. 



