Development of Dreissena polymorpha, Pallas. 163 
siderably increased in size, and already presents the appear- 
ance of the bivalve mussel-shell (fig. 1). This condition of 
the larva, apart from the processes which take place in the 
interior, continues for a long time. As it is peculiarly charac- 
teristic, and is useful for the discovery of the larva, a few 
sketches of it are given herewith. 
Hig. 1: 
Younger larva of Dreissena, with bivalve shell (s) and velum (vel.), seen 
from the side. «a, anus; m, stomach; mu, mouth; sm, adductor 
muscle, 
Before all things important and characteristic is, besides 
the shell, which is at first delicate and composed of a thin 
cuticular membrane, and subsequently somewhat stouter, the 
extensive velum. 
The velum appears as, so to speak, a fleshy organ, which 
is beset with stout cilia at its margin. It possesses a peculiar 
pigmentation, which greatly increases with the age of the 
larva (figs. l and 2). Fig. 1 shows the velum as seen from 
the side in a younger larva, and the following figures exhibit 
it in older larvee in different positions. From fig. 3, which 
represents an older larva seen from the side, it is evident what 
a considerable size the velum may assume. 
The larva is usually observed as represented in fig. 2. It 
swims on the surface of the water, with the velum directed 
upwards. In front and behind we see the valves of the shell 
projecting beneath the expanded velum (figs. 2, 3). The 
ciliary movement is most readily comparable to the rotation 
of a circular saw, since only a certain number of the cilia are 
extended simultaneously, while the interjacent ones appear to 
be bent. Sometimes the whole of the cilia are ‘stretched 
straight out. ‘The motion of the larva is a Hone, and 
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