86 



surface witli the byssal groove faces the right valve, and 

 it will be remembered that it is the right valve of the 

 shell that has the indentation known as the byssal notch. 

 Though there are no traces of the byssus in the adult 

 P. maximus, the byssus gland is very well developed^ It 

 is situated very deeply in the tissue, in fact practically 

 outside the foot, in the midst of the retractor muscle. If 

 a series of transverse sections is followed from the byssus 

 groove region of the foot to the retractor muscle, the 

 following sequence will be observed : — The byssal groove 

 is rather wider at the bottom, and this cavity runs in 

 towards the byssus gland. In sections taken below the 

 byssus groove the sides of the groove have coalesced and 

 the cavity alone is present. As we pass further in, the 

 dorsal wall of the cavity becomes ridged by longitudinal 

 projections, which gradually meet the ventral wall, so 

 that ultimately the original cavity is divided up into 

 compartments by parallel partitions running across from 

 the dorsal to the ventral wall (fig. 47, B. g., and fig. 9). 

 These compartments are deep and wide, but very narrow. 

 Sections showing this structure pass through the retractor 

 muscle alone, and are therefore posterior to the actual 

 foot itself. 



The partitions are composed of connective tissue in 

 which are to be found many muscle fibres, and are 

 bounded by a layer of epithelial cells almost cubical in 

 shape, and of course continuous with those of the byssal 

 gland. They are well provided with cilia. The 

 compartments terminate blindly, and at the same time 

 become reduced in width ; but at their blind ends, the 

 cells Cfig. 9 B. g. c.) are rather larger than the other 

 epithelial cePs and contain practically no contents. A 

 striictureless bodv, presumnblv the nucleus, is present in 

 each, but it stains differently from the other nuclei and 



