Develop^nent of Dreissena polymoiplm, Pallas. 163 



slderably increased in size, and already presents the appear- 

 ance of the bivalve ninsscl-shell (ti;^. I). This condition of 

 the hirva, apart from the processes which take phice in the 

 interior, continues for a long- time. As it is peciUiarly charac- 

 teristicj and is useful for the discovery of the larva, a few 

 sketches of it are given herewith. 



Fig. 1-. 



Younger larva of Dreissena, with bivalve shell (*) and velum {veL), seeu 

 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 (tigs. 1 and 2). Fig. 1 shows the velum as seen from 

 the side in a younger larva, and the following figures exhibit 

 it in older larva in different positions. From lig. 3, which 

 represents an older farva 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 very rapid, and 



11* 



