18 



opcb 



Figure 15: Schematic presentation of beating of the ciha of the velum of the veliger of Ostrea 



edulis L. (from Waller, 1981). 



ac — apical cilia; acb — aderal ciliary band; ipcb — internal preoral ciliary band; 



opcb — outer preoral ciliary band; pocb — postoral ciliary band; sh — shell. Arrows show the 



direction of beating of the waves of cilia. 



The speed of sinking increases when the ciUa of the velum stop beating. The 

 speed of sinking is greatest when the velum is rolled and the valves of the shell 

 closed. Rolling of the velum is affected by two or three pairs of larval muscles, 

 the velum retractors. They originate from the shell in the region of the pos- 

 terior part of the hinge line. In Dreissena polymorpha the dorsal retractor 

 passes along the dorsal side of the larva and supplies muscle fibers in the 

 dorsal region of the velum, which are attached to the velum integument. The 

 medial retractor proceeds forward from the place of its insertion to the ventral 

 and medial regions of the velum. One muscle is attached to the apical plate, 

 while the remaming fascicles are attached to various areas of the anterior part 

 of the velum (Meisenheimer, 1901). In Cardium edule there are individual 

 retractors for the medial and ventral regions of the velum (Creek, 1960). 

 During rolling of the velum first the apical plate withdraws into the shell, then 

 the lateral parts of the velum apparently roll up and the valves of the shell 

 close. During withdrawal of the velum under the shell, the stomach and esopha- 

 gus are shifted backward. Opening of the valves, slackening of the muscles, 

 and filling of the fluid in the extensive cavity of the velum, ensure its exten- 

 sion. 



The ascent of the veliger usually follows a spiral pattern; this type of 

 veliger locomotion is seen in Ostrea edulis, Mercenaria mercenaria, Dreissena 

 polymorpha, and others. In other species the veliger ascends in a vertical line, 



