274 CHARLES HOWARD EDMONDSON 



posed by that investigator, namely, that the organ serves as a 

 crushing surface for food collected between it and the epithelium 

 of the style sac. As there is no means by which food may enter 

 the style sac in species in which the style rests in a distinct 

 caecum, the opinion of Sabatier is not well founded. I have ob- 

 served the brown pigment in Mya to be most abundant during 

 the winter months and the cells nearest the typhlosoles are more 

 densely crowded with it. In cells remote from the typhlosoles 

 it is barely perceptible or may be entirely lacking. Since the 

 typhlosoles are highly vascular, I am of the belief that the pig- 

 ment may reach the cells from the circulatory system. 



Cross-sections of the middle and lower third regions of the style 

 sac indicate a gradual lessening in extent of the narrow cells of 

 the right typhlosole (fig. 10). The groove becomes more shal- 

 low and the typhlosoles less prominent as the distal extremity 

 of the style sac is approached. At the very distal tip of the style 

 sac transverse sections show the epithelium of the lining to be 

 greatly folded, while the groove and typhlosoles have lost their 

 identity (fig. 11). Longitudinal sections of the style sac repre- 

 sent the epithelium of the general surface of the tube to be thrown 

 into transverse folds which are low undulations in the proximal 

 portion of the tube, but become stronger and more irregular 

 ridges toward the distal extremity (figs. 12 and 13). 



Fig. 10 Transverse section of the style sac through the middle region. Ss, 

 style sac; G, groove of style sac; Lt, left typhlosole; Rt, right typhlosole. X 25. 



Fig. 11 Transverse section through the^distal extremity of the style sac show- 

 ing the folding of its inner wall. X 25. 



Fig. 12 Longitudinal section of the style sac in its proximal half. Bm, base- 

 ment membrane; C, cilia. X 120. 



Fig. 13 Longitudinal section of the distal half of the style sac showing the 

 increased size of the transverse folds of the epithelium over those of the preceding 

 figure. Bm, basement membrane; Tr, transverse folds of the epithelium of the 

 style sac. X 75. 



Fig. 14 A section of a group of cells of the general surface of the style sac. 

 X 280. 



Fig. 15 The gastric shield after removal from the left wall of Mya arenaria. 

 X7. 



Fig. 16 A transverse section of a fold of the left wall of the stomach to which 

 is closely applied the arms of the gastric shield; showing also the stratified nature 

 of the gastric shield. Gs, gastric shield. X 23. 



