15 



Figure 11: Frontal section through the veliger of Dreissena polymorpha Pall, at level of 



protonephridia (from Meisenheimer, 1901). 



eg — cerebral ganglion; es — esophagus; m — mantle; pg — pedal ganglion; 



pn — protonephridium; v — velum. 



henseni belonging to an unidentified species (Allen and Scheltema, 1972). A 

 larger velum facilitates larval migration to greater distances and efficient 

 collection of food particles (which are rare in open seas) since an increase in 

 velum size means an increase in number of ciliary bands. For example, in the 

 deepwater pholadid Xylophaga atlantica, the larva has no less than six ciliary 

 bands (CuUiney and Turner, 1976). 



Beating of the cilia produces the upward and somewhat forward move- 

 ment of the veliger, during which the velum is slightly inclined forward 

 relative to the axis of movement. While swimming, the veliger resembles a 

 flying helicopter. The principal locomotor organ is the outer preoral ciliary 

 band of the velum, consisting of a double row of ciliated cells. In the veliger 

 of O. edulis, 20-80 long cilia (50-70 |j.m in length) originate from each cell. 

 The cilia are in contact almost throughout their length and together form a 



