31 



midline of the foot in the region of the pedal ganglia. The byssus gland or, 

 more precisely, the byssus complex, comprises a series of glands producing 

 different secretions that facilitate the formation and attachment of byssus threads 

 to the substrate. The byssus threads of the pediveliger of O. edulis comprise 

 two strands, 3—7 [im in diameter, joined together and enveloped in a common 

 cover (Figure 27). In addition to the glands of the byssus complex, the foot of 

 the pediveliger may contain other glands (see Figures 23 and 28). In the 

 pediveliger of M. edulis and O. edulis, nine different types of glands have been 

 recorded in the foot; in Pecten maximus, there are five types of glands (Gruffydd 

 et al., 1975). Gruffydd and his colleagues divide the pedal glands of the 

 pediveliger into three groups: (1) glands producing very thin primary and 

 secondary byssus threads; (2) glands producing mucus on the tip and sole of 

 the> foot, which facilitates creeping; and (3) glands present but functioning only 

 after metamorphosis; and producing a secretion for attachment of the shell or 

 secondary byssus threads to the substrate (Table 2). The blanks in Table 2 are 

 probably due not so much to the absence of some glands, as to inadequate 

 study of the structure of the larvae. The number of cells in the glands of the 

 foot of the pediveliger vary in O. edulis from 10 to 250 (Cranfield, 1973). 



Cranfield (1973) has identified five phases in the behavior of the pediveliger 

 of O. edulis during settling. These, in his opinion, represent the sequence of 



Figure 27; Byssus threads of the pediveHger of Ostrea edulis L. (from Cranfield. IQ73). 



