narrow slit joins tlie sac over nearly its entire 

 length to the midgut; near the entrance to the 

 stomach the two structures are separated. The 

 sac is slightly twisted around the midgut and 

 occupies a somewhat dorsal position, while the 

 midgut forms the ventral portion of the common 

 structure (figs. 198, 199). A cross section of the 

 sac and midgut shows (fig. 204) that the two 

 channels are separated in the middle by a narrow 

 slit compressed by the two protruding lobes or 

 typhlosoles. In figure 204 the style sac is at the 

 top; its lumen is usually larger than that of the 

 midgut (lower part of the figure). This relation- 

 ship between the style sac and midgut is similar 

 to the topography of this organ in 0. chilensis 

 (Dahmen, 1923), 0. edulis (Yonge, 1926a), Mya 

 (Edmondson, 1920), Ensis (Graham, 1931a), 

 Mytilus edulis (Sabatier, 1877), M. latus and M. 

 magellanicus (Purdie, 1887), and Anodonta (Nel- 

 son, 1918). In the old literature the structure was 

 called a "tubular stomach" by Sabatier (1877), 

 and "pyloric appendix" by Purdie (1887), names 

 which have not been accepted in malacological 

 literature. 



Centimeters 



Figure 203. — Crystalline styles of C. virginica (left) 

 and C. gigas (right) in their natural position. Drawn 

 from live preparations. The wall of the stomach and 

 of the crystalline style sac has been dissected. 



J 



/ VT^-- 





& 



1T.5 



Mil I i meters 



Figure 204. — Transver.se section of the style sac (upper 

 part) and midgut (lower part). The crystalline style 

 is absent. Kahle, hematoxylin-eosin. 



The style sac is lined with densely packed 

 cylindrical cells that have large oval nuclei and 

 long cilia measuring about 20 ix. The intracellular 

 fibrillar apparatus is well developed. Phagocytes 

 and mucous cells are scarce. The basal membrane 

 rests on a thin layer of collagenous fibers; circular 

 muscles are sparsely arranged, as in the stomach. 



and there is no distinct muscular layer. The 

 epithelial cells of the two lobes (typhlosoles) of 

 the sac and midgut gradually change from robust, 

 long cells to shorter cells with smaller cilia, typical 

 for the lining of the midgut. The mucous cells 

 are more abundant in the midgut than in the sac. 



224 



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