Developme7\t q/'Dreisscna |)olynior[jli;i, Pallas. 1.59 



excretory organ which is so important for the Trochophora- 

 larva (E. Ziegler). With this we have the chief character- 

 istics of tlie Trochophoi-a, but this stage which repeats the 

 Trochophora in a niodilicd condition merely represents a 

 transitory period of tlic embryonic develo])mciit. In the case 

 of tlie IJnionidai the typical form of the Lanieliibraiich larva 

 receives even less expression. Here the velum is eiitircly 

 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 

 Unionidie likewise develop in tiie gills, and the larvaj which 

 are ex})elled from them, provided with shells and shell-hooks, 

 have already reached a tolerably advanced stage of develop- 

 ment — with the free-swimming larvaj of marine Lamelli- 

 branchs they have nothing in common. As is well known, 

 they attach themselves to fishes, upon which they live para- 

 sitically for a time f. This mode of life at once bears witness 

 to the extent of the adaptation which they have undergoi>e, 

 and which explains the striking transformation of their 

 organization as also the various modifications of their early 

 develoj)mental stages J. 



Since the development of the freshwater mussels which 

 have been mentioned exhibits so greatly modified conditions, 

 it seemed interesting to ascertain how Dreissena behaves in 

 this respect. As a near ally of the connnon mussel Dreissena 

 possesses more the chaiacter of a marine form, and we might 

 therefore expect to meet with free-roving larvae 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 Dreissena has been derived 

 from the region of the Euxine. 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 dieEutwickluugderUnioniden/'Denkschrift. 

 k. Akad. Wiss. zu Wien, Matb.-uaturw. CI. 45 Bd., 1889. 



t M. Braun, " Postembrvoiiale Entwicklung vou Anodonta," Zool. Anz. 

 1 Jabrg., 1878. 



Scbie3'bolz, op. cit. 



F. Scbmidt, " Beitrag zur Kenntiiiss der postembryonalen Entwicklung 

 der Najaden," Arcb. f. Ivaturgescb. 51 Jabrg., 1885."^ 



X A. Goette, " Bemerkmigen iiber die Embrj'onalentwicklung von 

 Anodonta 2)iscinaJis" Zeitscbr. f. ■\viss. ZooL 52 Bd., 1891. 



§ E. von Marteus, " Eine eingewanderte Muscbel," Der Zoologi.scbe 

 Garten, 6 Jabrg., 1865. 



