ANATOMY OF THE PEAEL OYSTER. 55 



/.' Below the oesophageal aperture the openings of three small sub-cesophageal ducts 

 (D.s.OB., fig. 7) can be readily made out, bringing the secretion of the anterior and 

 lower portion of the glandular organ. 



To see the openings of these ducts, divide the stomach horizontally along the mid- 

 lateral plane and syringe out the contents. Then a and b will he seen upon the 

 right side of the roof (see fig. 6), the remainder upon the ventral half (fig. 7). The 

 course of the larger ducts can also be traced for some distance by picking away the 

 glandular tissue around the stomach (see also figs. 8 and 9). 



Lining the greater portion of the gastric cavity, dipping into depressions and rising 

 over the folds, a gelatinous layer (corresponding to the " fleche tricuspide" of some 

 other molluscs) invests the epithelial lining. It is colourless and transparent ; and in 

 section is seen, under the microscope, to be a cuticular laminated structure in close 

 relation with the underlying epithelium. In freshly caught, health v individuals, the 

 distal end of the crystalline style is invariably seen protruding into the stomach from 

 the circular and larger anterior portion of the intestinal aperture. 



The intestine may be divided into three sections of approximately equal length, 

 namely : (a) The descending portion ; (b) the ascending portion ; and (c) the rectum 

 (see Plate VI., fig. 1). 



(a) The first or descending section of the intestine (Int. 1) passes ventrally, with a 

 slight inclination to the rear, into and through the posterior part of the visceral mass, 

 which is here composed of the tubules of the gonad. Its course then lies behind the 

 base of the byssal gland and between the converging bundles of the two pedal 

 retractor muscles. At this point it changes its direction and curves forwards and 

 downwards to the prominent antero- ventral corner of the visceral mass which marks 

 the point of its junction with the ascending branch (Int., 2). 



A longitudinal fold projects inwards from the anterior and one from the posterior 

 wall of the descending intestine. As these folds are vis-a-vis and to one side of the 

 median axis of the tube, they divide it into two unequal longitudinal chambers, or 

 rather gutters, that to the left being the larger (Plate VI., fig. 10, a). The two 

 folds are low and little prominent for a short distance from the stomach ; little by 

 little they reach further across the cavity till in the middle and lower thirds their 

 apices broaden and close together, so as to form Wo distinct tubes. The broader of 

 the two is circular in section at all points ; the smaller, except at the beginning, is 

 irregular in sectional outline and appears rather as a narrow and dee}) gutter along 

 the side of the broad cylindrical left portion (Plate VI., fig. 10, b). 



The narrow tube usually contains a train of partially digested food matter, while 

 under healthy conditions the larger cavity is completely filled with a clear gelatinous 

 solid cylinder, the crystalline style, a gently tapering, pliant and slightly elastic 

 rod. The right-hand tube, in spite of its insignificant diameter, is the true intestine, 

 the wider left being the sheath of the crystalline style, which here, as in Ostrea, 

 Pecten, Card mm and Mya, is imperfectly separated from the anterior portion of 



