20 



Figure 16: Cerebral, pleural (A) and pedal, visceral (B) ganglia of the pediveliger of Dreissena 



polymorpha Pall, (from Meisenheimer, 1901). 



ap — apical pit; bg — byssus gland; eg — cerebral ganglion; es — esophagus; gr — gill 



rudiment; hg — hind gut; m — mantle; mc — mantle cavity; pg — pedal ganglion; 



pig — pleural ganglion; sh — shell; v — velum; vg — visceral ganglion. 



which initiates shell formation of a veliger, i.e., the prodissoconch I. The shell 

 gland evaginates and soon a mantle forms consisting of a single row of cells. 

 The growing margins of the mantle envelop the larval body from the sides. The 

 shell bends along the mediodorsal line and becomes bivalved. 



The precise moment of initiation of calcification of the shell in bivalves 

 is not known. Evidently, this process begins after eversion of the shell gland, 

 as in gastropods. Due to the activity of the margins of the growing mantle, the 



