164 Dr. E. Korschelt on the 
generally takes place in such a way that the deeply pig- 
mented spot (p?), which is observable in fig. 2, is directed 
Fig. 2. Fig. 3. 
ee pi. 
Fig, 2,—Larva of Dretssena, looking down from above upon the velum, 
which is fully expanded. p, pigmentation of the velum (vel.); pi, 
pigment beneath the oral opening; s, shell, which is for the most 
part concealed by the velum, 
Fig. 3.—Older larva of Dreissena, with velum greatly expanded, seen 
from the side. m, retractor muscles of the velum (vel.); pi, pig- 
ment; s, the two valves of the shell. 
towards the rear, When swimming at full speed the larva 
suddenly halts, and for a longer or shorter time imparts a 
spinning motion to its velum 
without moving from the spot. 
At the smallest disturbance the 
velum is retracted, the valves 
of the shell, which are only 
slightly opened, close with a 
snap, and the larva sinks to the 
bottom. 
A striking feature is the 
bilobed character of the velum ; f 
this is especially noticeable at pt. ~  - 
the commencement of expan- 
sion, but is also visible in cer- 
tain positions in the fully ex- 
panded state (fig. 4). This 
condition reminds us strongly 
of the bilobed velum of the 
Gastropods. Attention has 
ure already been drawn Older larva of Dretssena, with expan- 
by Ziegler to a similar state “goa velum (ved.), which in the figure 
of things in the reduced velum is viewed obliquely. _p, pigmenta~ 
of Cyclas ; yet this condition is tion of the yelum; pr, pigmenta- 
not noticeable in Cyclas until pS Ee eel te mouth ; 
later stages of development, and “9 “°° O'S" 
Fig. 4, 
