276 CHARLES HOWARD EDMONDSON 



The circular folds of the style sac do not continue to the mar- 

 gin of the groove, but abruptly terminate at the lateral border of 

 the long, narrow cells on either side of the gi'oove. Therefore 

 the typhlosoles are not marked by undulating epithelium as is 

 the rest of the tube, but are smooth on their inner surfaces. 

 This latter feature is made plain by longitudinal sections, close 

 to the margin of the groove, in which the epithelium is seen to 

 rest upon a basement membrane disposed in an even line without 

 folds or undulations. 



5. THE GASTRIC SHIELD IN MYA ARENARIA 



Spreading over the left wall, the roof, and the floor of the pos- 

 terior half of the stomach of Mya arenaria is found the structure 

 which Nelson ('18) called the gastric shield. In this species the 

 shield does not cover the right wall of the stomach (fig. 18). 



The gastric shield consists of a cartilage-like layer applied to 

 the epithelium of the stomach wall growing thicker from the 

 posterior region forward where it develops into an irregular struc- 

 ture with three curved processes which clasp prominent folds of 

 the dorsolateral wall of the stomach (figs. 15, 16, and 19). 



Poli (1791) first described this organ under the name of 'fieche 

 tricuspide,' and suggested that it might control the flow of bile 

 into the stomach by the extension of its processes into the bile 

 crypts. 



Histological sections through the stomach wall and gastric 

 shield of Mya arenaria show the latter to be closely applied to 

 the columnar epithelium. Beneath the shield the epithelial cells 

 are not provided with cilia. It will also be observed that the 

 gastric shield consists of numerous strata comparable to the con- 

 centric layers of the crystalline style in cross-section. 



Fig. 17 Semidiagrammatic section showing the relation of the crystalline 

 style to the gastric shield in Mya arenaria and also to the proximal extremity of 

 the intestine. St, stomach ; Gs, gastric shield ; Cs, crystalline style ; Int, intestine. 

 X 2. 



Fig. 18 A transverse section of the stomach of Mya arenaria showing the 

 position of the gastric shield on the roof and the left wall of the stomach. St, 

 stomach; Gs, gastric shield; Ss, style sac. X 3^. 



