Development f?/"Drcis.scna polyrnovi)lia, PaUa.t. 1(57 



to ciiaMo it to olitaiu afconunodation witliiii tlic sli(;ll. In 

 the case of sucli larvn3 as have boon 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 J2:radiially 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 velnm. The latter, however, 

 gradually degenerates, and finally the only evidence of its 

 ])rcvions existence is the pigmentation of the anterior section 

 of the body, which is still retained for some time. 



A further cliange 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 papillaj, 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 Mijtilus by Lacaze-Duthiers *. It is difficult to 

 determine whetiier 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 pa})illa3, or whether they sprout 

 forth as real papilla?. At any rate a 

 few smaller papilla? are added behind 

 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 

 large dimensions and is capable of being 

 ])rotruded 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 

 vermiform-looking foot is stretched far 

 out (fig. 5), goes through the move- 

 ments of a feeler, and then fixes itself 

 by its extremity ; whereupon it contracts 

 and so drags the body after it. The 



* Lacaze-Dutliiers, " Meinoire sur le developpemeut des brancliies des 

 MoUusques Acephales Lamellibraiiclies," Anu. Sc. Nat. 4^s^r., Zool. t. v., 

 1866. 



Fiff. 5. 



Dorsal view of young 

 Dreissena crawling, 

 with foot (/) much 

 protruded. 



