Microns 



40 



Figure 208. — Cross section of a single digestive divertic- 

 ulum. Note the crypts of young cells at the corner of 

 the cross of the lumen. Kahle, hematoxylin-eosin. 



algae of larger size are ingested by phagocytes. 

 Yonge's work fully confirmed the idea, first ex- 

 pressed by Saint-Hilaire (1893), that the digestive 

 diverticula are the organs of absorption. He 

 found no evidence of any secretion from the di- 

 verticula and demonstrated the importance of 

 phagocytes in the digestive processes. Since the 

 work of the earlier investigators is fully discussed 

 by List (1902) and more recent investigations are 

 summarized in several papers of Yonge (1926a, 

 1926b), the reader interested in the history of the 

 problem is referred to these publications. 



The digestion of food also takes place in the 

 stomach where several digestive enzymes are 

 present. On the basis of our knowledge, which 

 admittedly is not complete, the process of di- 

 gestion seems to take the following course. After 

 being sorted several times by various mechanisms 

 of the gills and labial palps, the food particles 

 enter the stomach where the sorting continues 

 and the larger particles are broken by the com- 

 bined action of the crystalline style rotating 

 against the gastric shield and the chemical action 

 of enzymes which dissolve from the style. Very 

 small particles are pushed by the cilia, through 

 the ducts into the digestive tubules where they 

 are taken into the vacuoles of the digestive cells 



and are acted upon by the enzymes of these cells. 

 Usable material is ingested by the phagocytes or 

 is stored in the surrounding connective tissue. 

 Indigestible substances like colloidal carbon of 

 india ink are expelled. Some of the food particles, 

 especially of larger size, are engulfed by the 

 phagocytes which abound in the digestive tract. 

 Circulation of food in the ducts is maintained 

 by the ciliated cells. 



The stomach contains free enzymes which are 

 dissolved from the crystalline style. The most 

 active among them are the amylase and gly- 

 cogenase which digest starch and glycogen. 

 Yonge's experiments (1926a, 1926b) showed that 

 the optimum activity of oyster amylase is at 

 approximately pH 5.9. Purification by dialysis 

 or with absolute ethyl alcohol inactivates the 

 enzyme, but its action can be restored by the 

 addition of chlorides or broirddes. Besides these 

 two enzymes, the style contains a complete 

 oxidase system. The presence of oxidases in 

 the extract of styles was first demonstrated by 

 Berkeley (1923) in the Pacific coast clam, Sax- 

 idomus giganteus, in rock cockle, Paphia staminea, 

 and in soft-shell clam, Mya arenaria. This 

 finding lead Berkeley to advance a theory that 

 the crystalline style represents a reserve of oxygen 

 and is a factor in the anaerobic respiration of 

 mollusks. The theory is not supported by 

 sufficient evidence and has not been accepted by 

 the students of moUuscan physiology. 



The presence of oxidases in the styles of Ostrea 



Microns 



Figure 209. — Cross section of the duct leading to the 

 digestive diverticula. Kahle, hematoxylin-eosin. 



ORGANS OF DIGESTION AND FOOD OF THE OYSTER 



229 



