32 



hierarchic, fixed motor reactions leading to the final act of cementing. In the 

 absence of stimuli specific for a particular response, the latter is not produced 

 and the larva returns to an earlier phase of settling. In the opinion of Cranfield, 

 settling of bivalves and polychaetes is very similar. During settling the 

 pediveliger changes fi-om a ciliary to a muscular mechanism of locomotion 

 over the substrate; in this case the viscosity of the mucopolysaccharide secre- 

 tion mcreases and the secretion of byssus threads begins. At the end of settling, 

 a secretion is produced which cements the larva to the substrate. The first 

 exploratory stages of behavior of the pediveliger during settling were described 

 above. In the pediveliger of O. edulis, later, in the course of settling, the 

 muscular movement of the foot becomes sharper and stronger and the speed 

 of locomotion decreases. The body turns occur more often in locomotion. 

 Initially, the larva creeps in a straight line, rarely turning. Subsequently, 

 movement becomes zigzag. With an increase in fi-equency of turning, the angle 

 of the path of movement decreases; the track of a pediveliger resembles a star 

 and then a circle. The last phase of settling — cementing of the pediveliger 

 of an oyster commences with a turn to the right (all previous turns have been 

 to the left), the shell rests on the foot, and the larva rotates on the foot, which 

 is attached to the substrate by its tip, and the left valve. Similar processes were 

 observed by Coon and Borne (1985) during settlement of the pediveliger of 

 Crassostrea gigas. The larva, ready to settle, after having swum with con- 

 tracted legs, now begins to extend its legs forward during swimming. It then 

 drops to the bottom and moves over the substrate, executing a series of sliding 



Table 2: Pedal glands of pediveliger of various bivalves 

 (from Gruffydd et al., 1975) 



Species 



Mucous glands 



Glands of furrows 



Glands of primary 







(tentatively producing 



byssus 







and attaching 



secondary 



threads 







byssus threads) 





Nucula delphinodonta 



- 



. 





+ 



Mytilus edulis 



+ 



+ 





+ 



Pecten maximus 



+ 



+ 





+ 



Crassostrea virginica 



- 



+ 





+ 



Ostrea edulis 



+ 



+ 





+ 



Musculium securis 



- 



- 





+ 



Sphaerium notatum 



- 



- 





+ 



Dreissena polymorpha 



- 



- 





+ 



Lasaea rubra 



+ 



+ 





+ 



Chione cancellata 



- 



- 





+ 



Mercenaria mercenaha 



- 



- 





+ 



Venerupis pullastra 



- 



- 





+ 



Venus striatula 



- 



- 





+ 



Xylotrya gouldi 



+ 



- 





+ 



