164 



Dr. E. Koischelt on the 



generally takes place in such a way that the deeply pig- 

 mented spot ( in) , which is observable in fig. 2, is directed 

 Fig. 2. Fig. 3. 



uJi 



Fig, 2. — Larva of iJreissena, louldug dowu from above ujion the velum, 

 whicli is fully expanded, p, pigrmentation of the velum {veJ.) : 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, w?, retractor muscles of the velum {vel.) ; pi, pig- 

 ment ; s, the tsvo 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 



witliout moving from the spot. 



At the smallest disturbance the 



velum is retracted, the valves 



of the shell, wdiich are only 



slightly opened, close with aT"*""; 



snap, and the larva sinks to the 



bottom. 



A striking feature is the 

 bilobed character of the velum ; 

 this is especially noticeable at-j^i^-- 

 the commencement of expan- 



Fi?. 4. 



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 



I'd. 



moreover already been drawn ,^,i i„,. .„„(? n.. ,.-..L..^ ,..^fi, w .., 

 , r/. , J . .. Ulder larva 01 X'/vjAsf^rt, with expim' 



by Zieglcr to a similar state ded veluuutv/.). which in the ligure 



of things in the reduced velum is viewed obliquely, p, pigmenta- 



of Ciiclas ; yet this condition is ti^^" "-'^ the velum: pi, pigmenta- 



uot noticeable in Cudas until ^'^'l] "^ ^Y ''T/r' f n'' '"""^^ ' 



1 . ^ r 1 1 1 ■*• tlie valves ot the shell. 



later stages oi development, and 



