CRYSTALLINE STYLE OF LAMELLIBRANCHS 71 



The epithelium of the stomach is thrown into many folds and 

 ridges, with deep grooves passing between them. Figure 16 

 gives the appearance of the stomach of Anodonta when opened 

 along the right margin of the dorsal wall, and spread out fiat. 

 It also shows the relation of the two orifices, opening side by side 

 and separated by the typhlosole, and the large deep groove lead- 

 ing across the wall of the stomach and entering the intestine 

 proper. 



In some forms a portion of the stomach is more or less sepa- 

 rated by a constriction, forming, in Anodonta, a pouch-like di- 

 verticulum on the dorsal side (fig. 1) or a distinct caecum ex- 

 tending the length of the stomach, beneath its ventral wall, as in 

 Modiolus. This blind sac performs in the latter genus, a very 

 important function, as will be shown later. 



The gastric shield 



On the anterior wall of the stomach, at the point where the 

 style comes in contact with the mucosa is a singular structure 

 which I have called the gastric shield. This is a thin, plate-like 

 sheet, as clear as glass, and of the consistency of cartilage. In 

 an adult Anodonta it measures approximately 8 x 5.5 mm. in its 

 greatest extent. It assumes many diverse forms in different 

 species, but in general I find three main lobes, and a blunt 

 apical projection (fig. 11). The edges of these lobes are firmly 

 held between the folds of the epithelium, and, in some species at 

 least, the border of the shield dips down into the crypts of the 

 hepatic ducts. In some instances a sharp spur of the shield may 

 run for some distance into the orifice. 



The cells beneath the shield are, according to Gutheil ('11) 

 the only columnar cells of the entire alimentary tract which are 

 devoid of cilia. Their function is to secrete droplets of a color- 

 less matter which harden to form the shield. 



This structure is closely applied to the epithelium lying be- 

 neath it, and, as it rests between folds of the stomach wall, it 

 presents an outer surface concave in outline. In Venus an out- 

 ward projection of the epithelium forms a sort of tubercle which 



