Centimeters 



Figure 200. — Gastric shield viewed in its natural position 

 in a dissected stomach. The position of the crystalline 

 style is indicated by the dotted line. Drawn from an 

 unpreserved preparation. 



muscle fibers may be found under the basement 

 membrane. In general, the histological picture 

 of the stomach of an adult C. inrginica is similar 

 to that described for the spat of this species by 

 Shaw and Battle (1957), C. angulata by Leenhardt 

 (1926), and 0. chilensis and 0. edulis by Dahmen 

 (1923) and Yonge (1926a) respectively. 



GASTRIC SHIELD 



The stomach wall in front of the openings to 

 the midgut and style sac is covered by a thin 

 but tough, irregularly shaped membrane (fig. 200) 

 made of translucent and slightly striated material. 

 The structure, named the gastric shield by Nelson 

 (1918), rests on a prominent epithelial ridge of 

 narrow colunmar cells with oval nuclei, rich in 

 chromatin (fig. 202). The cells are devoid of 

 cilia. The shield is made of two portions of 

 different size, joined together by a narrow middle 

 piece (fig. 200). The thicker portion of the shield 

 lies over the peak of the ridge and is underlined by 

 tlie tallest cells in the area. On both sides of 

 the peak the epithelium flattens and at the edges 

 changes into the typical ciliated lining of the 

 stomach. The surface of the sliield is roughened 

 by the remnants of food particles embedded in it. 



The origin of the shield has not been fully 

 e.xplained. Obviously, it is tlie product of the 

 underlying cells, but the process of its formation 

 has not been studied sufficiently. One view, 

 advanced by Gutheil (1912) and shared by some 



investigators, assumes that the shield is formed 

 by the droplets secreted by the epithelial cells. 

 No evidence in support of this view can be found 

 in the histological preparations of the stomachs 

 of 0. edulis and C. virginica. No droplets could 

 be seen in the sections of stomach, and no other 

 indication of the secretory activities of these cells 

 could be found. Yonge (1926a) thinks that the 

 shield is very likely formed by the fusion of the 

 cilia and in support of this view points out that 

 the structure is attached to the epithelium by fine 

 threads which transverse the substance of the 

 shield and resemble the cilia. Indistinct trans- 

 verse striation can be seen in the sections of the 

 stomachs of C. mrginica fixed in osmic acid and 

 stained with iron hematoxylin (fig. 202). The 

 question could be settled by electron microscopy, 

 which would reveal the structure of the cilia if the 

 latter are present within the shield substance. So 

 far no such studies have been made. 



The shield is not destroyed by boiling in a 40 

 percent solution of potassium hydroxide. Treat- 

 ment with iodine followed by a strong solution of 

 zinc oxide gives the deep violet coloration that is 

 characteristic of the color reaction for chitin 

 (Zander reaction) . These facts support Berkeley's 

 (19.35) findings that the material of the shield of 

 the common Pacific coast clam, the Pacific gaper 

 {Schizothaerus mdalli nutalli Conrad), is made of 

 chitin and contains no chondrinlike constituent. 



In C. angulata Leenhardt (1926) described the 

 torch bearing cells near the edges of the area 

 occupied by the gastric shield. The function of 

 the cells is not known. They are not found in my 

 preparations of C. virginica and are not mentioned 



Microns 



Figure 201. — Cross section of the wall of the stomach. 

 Kahle, Hematoxylin-eosin stain. 



222 



FISH AND WILDLIFE SERVICE 



