onstrated that the styles of four species of bivalves 

 (Crassostrea gigas, Mya arenaria, Schizothaerus 

 nuttalii, and Saxidomus giganteus) in addition to 

 protein, yield, on acid hydrolysis, glucionic and 

 sulphuric acids and a hexamine, the essential 

 constituents of both mucin and chondrin. The 

 ease of the hydrolysis and the solubility of the 

 style materials indicate that mucin rather than 

 chondrin is involved. The variations in the solu- 

 bility and the quantitative differences in the 

 chemical composition of the styles suggest, ac- 

 cording to Berkeley, that the less readily soluble 

 styles contain larger quantities of mucin. 



All examined styles are carriers of certain 

 enzymes which they yield upon dissolution. The 

 role the styles play in digestion is discussed later 

 (see p. 230 of this chapter). 



MIDGUT AND RECTUM 



The portion of the intestine between the stomach 

 and the rectum is called the midgut. It begins 

 at the ventral wall of the stomach next to the 

 opening of the crystalline sac and runs parallel to 

 the sac as far as its distal end, then tui'ns sharply 

 backward parallel to its previous com'se (fig. 197, 

 int.). The ascending branch of the intestine 

 makes a loop that completely encu-cles the stomach 

 and continues as a descending branch which ends 

 with the rectum and anus (r., a.). 



Throughout its entire length the midgut is 

 characterized by a well-developed typhlosole 

 which extends along its inner wall (fig. 205). 

 The gut is lined with columnar ciliated epithelium; 

 there is an abundance of mucous cells and of 

 wandering leucocytes. The muscular layer is 

 absent. 



The rectum (fig. 197, r.) runs along the dorsal 

 side of the heart. In this respect the oyster 

 differs from many other bivalves (sea mussels, 

 clams, fresh-water mussels) in which the rec- 

 tum runs through the heart. The structure of 

 the rectum is similar to that of the midgut; the 

 main difference is the disappearance of the well- 

 developed typhlosole near the anal region where 

 the lining is thi-own into numerous small folds 

 (fig. 206). A distinct feature of the rectum is a 

 circular layer of smooth muscles which, however, 

 do not form a sphincter at the anus (fig. 207). 

 According to Leenhardt (1926), the anal sphincter 

 is present in the Portuguese oyster. The surface 

 epithelium of the anal region is well developed 

 and abounds in mucous cells. 



DIGESTIVE DIVERTICULA 



The stomach is surrounded by an irregularly 

 shaped mass of dark tissue which has been called 

 the "liver" or "hepatopancreas." Its color varies 

 from light yellow to dirty green and dark brown. 

 In most cases the color is noi visible through a 

 white or creamy layer of connective tissues rich 

 in glycogen. 



Yonge (1926b) has shown that assimilation and 

 intracellular digestion takes place in this mass of 

 darkly colored tissues; it has none of the known 

 functions of the liver or pancreas. He named it 

 "digestive diverticula", a term that correctly 

 describes its role. 



The digestive diverticula are made of a large 

 number of blind tubules emptying into several 

 large ducts which lead to the interior of the 

 stomach. The structure of the tubules is simple. 

 In cross section (fig. 208) they are usually round 

 with a lumen in the form of a cross. The tubules 

 are surrounded by connective tissue in which 

 muscle fibers are absent. The digestive cells 

 which form the interior of a tubule are large and 

 well vacuolated, with large nuclei at their base. 

 Food vacuoles can be seen in them during feeding. 

 At the corners of the "cross" of the lumen one 

 usually finds crypts of young cells with dark 

 staining protoplasm, large and compact nuclei, 

 and indistinct cell boundaries. Cells from these 

 crypts replace the old cells that are cast off. The 

 digestive cells of the American oyster are non- 

 ciliated, but the cells in the diverticula of other 

 bivalves {Teredo) have been reported to have 

 ciha (Potts, 1925). Yonge (1926a, 1926b) believes 

 the cilia are present in the tubules of edulis but 

 probably retract so rapidly that they cannot be 

 seen in the fragments of tissues pressed by a 

 cover slip. I was not able to detect them in C. 

 virginica. Phagocytes are very abundant between 

 the cells and in the surrounding connective 

 tissue. 



The ducts that connect the tubules with the 

 stomach are circular in cross section and are 

 fined with cifiated epithelium (fig. 209). Their 

 lumen is, however, irregular due to the variations 

 in the height of the epithelial cells. The epithe- 

 lium is similar to that of the stomach and con- 

 tains many mucous glands and phagocytes. 



ALIMENTARY TRACT AND FORMATION 

 OF FECES 



Food ingested by the oyster is moved through 

 the alimentary canal by the ciliary action of the 



226 



FISH AND WILDLIFE SERVICE 



