158 Dr. E. Korsclielt on the 



miglit ill Its case expect to find free-roving larvae (in fresh 

 water). Ciiiefly on this account 1 endeavoured during tlie 

 suninior of last year to elucidate the hitherto unknown repro- 

 duction and development of this mollusk. At that time my 

 investigations were undertaken too late ; hut with the com- 

 mencement of spring of the present year I recommenced my 

 observations and persevered with them until I finally suc- 

 ceeded in deciding this question. I would remark at the 

 outset that in the following pages T have no intention of 

 giving a detailed account of the development of Dreissena. 

 J\ly observations were, as I have already hinted, directed 

 chiefly towards the one point of the presence or absence of 

 free-roving larvfje. Moreover they were really intended to 

 partake of an orientating character, in order to determine the 

 time and manner of the reproduction of Dreissena, and thereby 

 to facilitate a subsequent minute investigation of the deve- 

 ]o})ment, since this was not possible for me this year on 

 account of other tasks which could not be postponed. Mean- 

 while it is already possible to determine the leading features 

 of the development of Dreissena, so that a communication on 

 the subject will not be unwelcome. 



As a general rule develo})ment by means of free-roving 

 larvae provided with a velum is characteristic of the Lamelli- 

 branchs. Nevertheless an excejition is formed by the fresh- 

 water mussels. As is frequently the case in freshwater forms, 

 e.g. in the Annelids, Turbellarians, &c., these creatures adopt 

 a direct development with the freshwater existence, and 

 abandon the original method by means of free-roving larvoi. 

 Thus we see that the freshwater mussels have already 

 assumed the adult form on leaving the mother {Ci/clas, Fisi- 

 diuni) or else pursue a course of development in which larva} 

 indeed appear, but which is at once recognizable as having 

 undergone secondary modification and considerable adajitation 

 to the present mode of life of the mussels (Unionids). 

 Cyclas and risidhnn exercise a brooding-process, since they 

 form a kind of brood-pouches within the gills, in which the 

 eggs develop until the formation of the perfect animal is 

 almost complete. Nevertheless these mussels pass through 

 a stage in which the embryo is to be considered equivalent 

 to the free-roving larva, which has been compared to tiie 

 IrocJiop/iora of the Annelids. The velum of the larviB of 

 marine mussels is indicated by an area of cilia in the embryo 

 of Ctjclas ; in this case therefore we may speak of a rudimen- 

 tary velum*. Moreover it possesses a primitive kidney, the 



* E. Zicglcr, ''Die EntwicUliinp- von Ci/clas contrn, Lniu.,"' Zeitsdir. f. 

 wiss. Zool. 41 B(l., 188-"). 



