8s JOURNAL OF CONCHOLOGY, VOL. II, NO. 3, JULY, 1904. 



features in the larval organization are recognizable from fig. i, — the 

 bivalve shell (figs. 2 — 3, s.), the velum with its retractors, and a 

 ciliation, also found in the larvae of other mussels, in the neighbour- 

 hood of the anus. As in the Trochophora, the oral opening lies 

 behind the powerful zone of cilia on tlie velum. In older larvae there 

 appears behind the mouth a peculiar pigmentation, which often has a 

 bilobed appearance (figs. 2-4, //.), which I was at first inclined to 

 regard as the earliest indication of the byssus-gland; the position of 

 the latter, however, would not well agree with this. A closer investiga- 

 tion of the subsequent stages will be necessary before we can decide 

 whether a more important significance attaches to this formation." 



I'lS'. 4. 



Fig. 1.— Older larva o( Dreissensia, with expanded velum (vei.), which in the figure is viewed 

 obliquely ; /, pigmentation of the velum : //, pigmentation in the vicmity of the mouth ; 

 s, the valves of the shell (from Korschelt, '92). 



The larvae may be looked for at the end of June and early part of 

 July. They swim about for eight days or so, feeding apparently upon 

 minute floating algte; during this pelagic existence the foot makes its 

 appearance, and shortly after the larva sinks to the bottom for good. 

 The velum then gradually disappears; the mantle and gills develop; 

 the foot attains larger dimensions, and is capable of being protruded 

 a long way from the shell, and the young mussel now progresses solely 

 by the aid of this vermiform organ, which, when stretched out, goes 

 through the movements of a feeler, and fixing itself by its extremity, 

 contracts and so drags the body after it. The foot has now become 

 an organ of locomotion in place of the lost velum, and by its aid the 

 animal crawls actively about amongst the stones and mud, and over 

 the waterweeds, and thus consequently passes through a second freely 

 mobile stage. 



