168 On the Development of Dreissena polymorpha. 
mussel, provided with an almost circular shell, might at this 
stage be taken for a young Cyclas. The velum has degene- 
rated, and in the foot it now possesses a serviceable organ of 
locomotion, by aid of which it crawls actively about. The 
mussel consequently passes 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 Dretssena-larve 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 (EH. von 
Martens, loc. cit.), and nevertheless we find Dretssena so 
widely distributed and occurring in many places in enormous 
quantities. I have shown above that the larvee rove about 
for a pretty long time. If therefore they happen to be in a 
stream they will be able during this 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 their being attached to vessels and by being 
transported with timber, &c. The retention of the free-roving 
larvee was doubtless of the greatest importance to Dredssena. 
I have already emphasized the fact that owing to the reten- 
tion of these primary larve Dreissena differs in a striking 
degree from other freshwater forms. 
* In the discussion on the present subject Prof. von Martens alluded to 
an observation by Rossmiissler, according to which Dreissenu retains a 
certain power of mobility, in that it is able to detach itself from its base 
and fix itself again at a greater or shorter distance. Although the foot, 
when contrasted with its size in the young state (fig. 5, 7), has very 
greatly degenerated in the 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 opportunities of observing the way in which 
individual specimens, which I had detached 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 days only a very short space is 
traversed. In so far as I took notice of this point the utilization of the 
foot as an organ of locomotion appeared to be very incomplete. 
That the foot retains a certain capacity for locomotion would, more- 
over, follow from the observation communicated by Reichel (Zool. Anz, 
x. 1887, p. 488), according to which in winter Dreissena retires into deeper 
water from the shallower parts near the shore. If this is so, it follows 
that the byssus, with which the animals are attached, must be thrown 
off. It is worthy of note in connexion with this statement that Drets- 
sena is always found in masses, and the individuals must therefore come 
together again after having previously separated. 
