30 LAND AND FRESH-WATER SHELLS. 



There can be little doubt that the mother-substance of glycogen 

 in the Vertebrates, at any rate, is carbohydrate food, while the 

 ingestion of proteid matter seems also to favour its production, 

 but in a far less degree. The former kind of foodstuff (starch) 

 is in them changed by peculiar diastatic ferments — the ptyalin of 

 the saliva, and the amylopsin of the pancreatic juice — into sugar, 

 which is taken up from the digestive tract, as such, by the radicles 

 of the portal vein to be carried to the liver, there to undergo, by 

 a process of dehydration, its conversion into glycogen. The latter 

 — proteid matter — is, in them, converted into peptones by the 

 action of the pepsin (in the presence of hydrochloric acid) of the 

 gastric juice, and the trypsin (inthe presence of sodium carbonate) 

 of the pancreatic juice, and afterwards split up in the liver into 

 a non-nitrogenous portion (glycogen) and a nitrogenous portion 

 (probably urea). We have thus a starch diet and an albuminous 

 diet, producing glycogen in the liver of vertebrates. Where does 

 the analogy exist in the case of the Mollusca? Their food is 

 essentially, in the majority of cases, a vegetable one, and forming 

 the chief chemical constituents of plants are carbohydrates and 

 proteids. The carbohydrates are starch, inulin, dextrin, and 

 sugars, including glucose, cane-sugar, and various others. 

 Proteids are there as protoplasm, aleurone grains, crystalloids, 

 gliadin, vegetable fibrin, and a native albumen which is soluble in 

 water, and coagulable t^ heat, and, in many respects, identical 

 with animal albumen. And we have previously spoken of the 

 action, in the Mollusca, of the salivary secretion and of the 

 amylolytic ferment of the secretion of the " Mitteldarmdriise " 

 in converting starch into sugar; and we have already spoken of 

 the action of the trypsin of the secretion of the " Mitteldarm- 

 driise in the Mollusca, as converting colloid proteids into crystalloid 

 peptones. And that, in the Mollusca, glycogen is immediately 

 formed from the ingesta, has been proved by Barfurth, who found 

 that after three weeks' fasting it had disappeared from the liver 

 of Helix, but that it reappeared in from nine or ten hours after 

 feeding, and by Hammersten in the decrease in the amount of 

 glycogen showed (already noticed) in the liver of H. pomatia^ 

 which had hibernated in a warm room. There seems, then, every 

 reason for us to believe, in the face of no evidence to the contrary, 



