NUTRITION 



97 



minutes. In other cases, as in Mya (Edmondson), it is per- 

 manent and may take months to regenerate if excised. As the 

 hepato-pancreatic secretion which is poured into the stomach 

 contains a proteolytic enzyme which rapidly dissolves the 

 style in vitro, the permanence or otherwise of the style 

 possibly depends (Yonge) simply on whether it is protected by 

 enclosure in a separate diverticulum or lies exposed in an open 

 groove of the intestine. Mitra (1901) showed that extracts 

 of the style have a strong amylolytic reaction. This is abun- 

 dantly confirmed in a large number of genera. Style extracts 

 do not digest fats, proteins, inulin, cellulose, or cane sugar. 

 They break down starch and glycogen completely with the 

 production of glucose. The temperature-optimum (32° C.) is, 

 as would be expected, lower than that of the ptyalin in the 

 saliva ; on the other hand, the amylase found in the digestive 

 gland of the lobster has a very high optimum — about 56° C, 

 according to Yonge (1924). Temperature optima in enzyme 

 reactions deserve further inquiry as a limiting factor in geogra- 

 phical distribution. 



In connection with the digestion of carbohydrates there 

 is one point which will merit further investigation. Cellulose 

 is an important ingredient in the diet of all animals living on 

 plants, yet in vertebrates no cellulose- splitting ferment has 

 been identified with certainty. Biedermann and Moritz (1898) 

 found a cellulose-splitting ferment in the digestive gland of 

 the snail ; and the same authors also detected a cytase in the 

 hepatopancreas of Astacus ; but Yonge (1924) was unable to 

 detect any cellulose-splitting action in extracts of the diges- 

 tive gland of the Norwegian lobster. In herbivorous mammals 

 cellulose splitting appears to be effected by micro-organisms 

 living symbiotically in the alimentary tract. 



For a detailed account of intermediate carbohydrate meta- 

 bolism other sources must be consulted. However, mention 

 must be made of the part played by the pancreas in verte- 

 brates. The mammalian pancreas contains, in addition to 

 the exocrine acini, groups of cells known as " islets of 

 Langerhans." In Teleosts the islet tissue is wholly or partly 

 separate (Rennie) from the acinar elements. In 1889 Mering 



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