25 



Figure 21: Pediveliger of Mytilus edulis L. ( from Bayne, 1971). 



f — foot; o — osphradium; rpr — renopericardial rudiment 



Remaining legend same as in Figure 20. 



on each gill rudiment (Bayne, 1971, 1976). During creeping, gas exchange is 

 facilitated by the movement of cilia on the foot and of the foot itself The 

 circulatory and excretory systems do not change substantially during develop- 

 ment from veliger to pediveliger. 



Locomotion : In the pediveliger stage the velum, the initial organ of loco- 

 motion, attains maximum development, partly in connection with the increase 

 in shell mass and in larval body size. In addition, the new locomotor organ — 

 the foot — ^begins to fiinction. Like most larval organs, the foot, too, is a 

 multifunctional organ. Its primary fiinction is to probe the substrate for settling 

 and attachment of the larva by means of byssus threads or a cementing secre- 

 tion. The foot develops as an ectodermal outgrowth on the ventral side of the 

 body between the mouth and the anal pore (see Figures 10 and 13). The heel 

 of the foot (metapodium) begins to develop early and is covered with short 

 cilia. Ventral to it lies the toe (propodium), which later becomes larger than 

 the metapodium. Between the propodium and metapodium lies a depression in 

 which the duct of the byssus gland opens. A fiirrow, extending the entire length 

 of the foot, divides it into two equal halves (Figure 22). 



