16 



cicpn 



Figure 12: Protonephridium of the veliger of Dreissena polymorpha Pall, in longitudinal 



(A) and transverse (B) section (from Meisenheimer, 1901). 



clcpn — canalicular cell of protonephridium; clpn — canaliculus of protonephridium; 



cpn — cilia of protonephridium; tcpn — terminal cell of protonephridium. 



"complex cilium". The orientation of cilia in the complex cilium, parallel or 

 perpendicular to the plane of beating, ensures a thrust of maximum force, 

 moving the veliger upward. The efficient beating of straight cilia is directed 

 downward and coordinated by the wave of beating by the entire band (Figure 

 15). The reverse upward motion of cilia is accomplished in a bent condition. 

 If the wave passes at a right angle to the plane of beating of an individual 

 complex cilium, then the veliger, moving upward, will rotate in the direction 

 opposite to that of the wave. Inhibition of this rotation is ensured by beating 

 cilia that are inclined relative to the beating ciliary band and relative to the 

 velar margin. 



The postoral ciliary band of the velum also plays some role in the loco- 

 motion of the veliger. It is a single row of complex cilia 1 5—20 )im long. Each 

 complex cilium consists of four to five simple cilia. Their beating is effectively 

 directed upward and counteracts the beating of the cilia of the preoral ciliary 

 band (pcb) (Waller, 1981). 



At an average speed, the veliger expends nearly 10% of its energy for 

 locomotion. Locomotion at higher speeds increases this expenditure up to 50% 

 (Zeuthen,1947). 



The veliger moves upward with a ftjlly extended velum and intensive cilial 

 beating. As the intensity of cilial beating decreases, the veliger slowly sinks. 



