Development of Dreissena polymorpha, Pallas. 167 
to enable it to obtain accommodation within the shell. In 
the case of such larve as have been collected at the bottom of 
the aquarium we observe how the foot is extended like a feeler 
when the animal is at rest, while the shell slightly opens. 
The velum is then gradually unfolded, and the larva swims 
away by its aid, to all appearances quite as actively as before. 
The foot, which is already tolerably developed, is therefore 
present in addition to the velum. The latter, however, 
gradually degenerates, and finally the only evidence of its 
previous existence is the pigmentation of the anterior section 
of the body, which is still retained for some time. 
A further change in the larva results from the greater 
development of the mantle. This arose in the shape of a 
fold, which advanced with the shell from the dorsal towards 
the ventral surface. It now comes into greater prominence, 
since it protrudes at many points on the free edges of the 
shell. Between the mantle-fold and the foot the rudiments 
of the gills were formed. They appear at this time as a few , 
tolerably large laterally compressed papille, the approxi- 
mated edges of which are beset with strong cilia, so that the 
observer seems to be looking at richly ciliated clefts. The 
aspect which they present at this stage is very similar to that 
described for Mytilus by Lacaze-Duthiers *, It is difficult to 
determine whether the rudiments of them are developed in the 
shape of a fold, which becomes notched at a very early period 
and so produces the supposed papille,; or whether they sprout 
forth as real papille. At any rate a 
few smaller papillae are added behind Fig. 5. 
to the large ones in front, so that the 
rudiment of the gills is continued pos- 
teriorly in an undulating line, which 
might perhaps be regarded as the free 
edge of a fold. 
Meanwhile the foot has attained very 
Jarge dimensions and is capable of being 
protruded a long way from the shell. 
‘The young mussel, for such it must by 
this time be termed, now progresses 
solely by its aid. We notice how the 
vermitorm-looking foot is stretched far ie erie nae 
out (fig. 5), goes through the move- “Dovey opablime 
ments of a feeler, and then fixes itself with foot (f) much 
by its extremity; whereupon it contracts protruded. 
and so drags the body after it. The 
* Lacaze-Duthiers, ‘‘ Mémoire sur le développement des branchies des 
Mollusques Acephales Lamellibranches,” Ann. Sc. Nat. 4° sér., Zool. t. v., 
1856. 
