168 On the Development 0/ Dreissena polyraorpha. 



mussel, provided with an almost circular shell, might at this 

 stage he taken for a young Cydas. Tiie velum has degene- 

 rated, and in the foot it now possesses a sennceable organ of 

 locomotion, by aid of which it crawls actively about. The 

 mussel consequently ])asscs through a second freely mobile 

 stage. While the body of the young mussel increases in 

 bulk, the growth of the foot is retarded, and thus acquires the 

 stump-like form which it possesses in the adult. The mussel 

 finally becomes fixed '^. 



The ready mobility of the Dre{ssena-\a.x\2d has doubtless 

 contributed materially to the dissemination of the mussel, 

 which has advanced with quite astonishing rapidity. It is 

 extremely probable that its immigration into Germany did 

 not take place until the second decade of this century (E. von 

 Martens, he. ci't.), and nevertheless we find Dreissena so 

 widely distributed and occurring in many places in enormous 

 quantities. I have shown above that the larva) rove about 

 for a }»retty long time. If therefore they happen to be in a 

 stream they will be able during tliis interval to be carried 

 forward for a long distance by the current. Von Martens 

 has shown how the advance of the mussels up stream is 

 brought about by tlieir being attached to vessels and by being 

 transported with timber, &c. The retention of the free-roving 

 larva? was doubtless of the greatest importance to Dreissena. 

 I have already emphasized the fact that owing to the reten- 

 tion of these ])rimary larva? Dreissena ditfers in a striking 

 degree from other freshwater forms. 



* In the discussion on the present subject Prof, von Mai'tens alluded to 

 an ol:)ser^•ation by Eossmiissler, according to which Dreisseyta retains a 

 certain ])O^Yer of mobility, in tliat it is able to detach itself from its base 

 and fix itself again at a greater (ir sliorter distance. Although the foot, 

 ■R-hcn contrasted with its size in the younjr state (fig. •'), f), has very 

 greatly degenerated in tlie adult, it nevertheless appears to be used as an 

 organ of locomotion, although to a limited extent, at a later stage of life, 

 I myself indeed had many op])ortunities of obserA ing the way in which 

 individual specimens, which 1 had detaclied from a large mass, climbed 

 high up the side of the aquarium and there fixed themselves. In the 

 case of such isolated individuals this appeared especially to occur when 

 the water in course of time began to go bad. The motion is, however, 

 very slow, and in the course of several diiys only a very short space is 

 traversed. In so far as I took notice of this point tlie utilization of the 

 foot as an organ of locomotion a]ipeared to be very incomplete. 



That the foot retains a certain capacity for locoumtiou would, nuire- 

 over, follow from the observation connmuiicated by Keichel (Zool. Anz. 

 X. 1887, p. 488), according to which in winter Drvussena retires into deeper 

 water from the shallower parts near the shcre. If this is so. it follows 

 that the byssus, with which the animals are attached, must hv thrown 

 oil". It is worthy of note in connexion witli this statement that Dreis- 

 sena is always found in masses, and the individuals must therefoiv come 

 together again after having previously sepamted. 



