ANATOMY AND PHYSIOLOGY OF A SNAIL. 29 



epiphragm 10^26 per cent. The liver secretion is acid to litmus 

 paper. In Planorbis, Limncea, and Buccinum the oesophagus has 

 a lateral caecal crop appended to it. The anus terminates on the 

 left side of the body in Planorbis, and it has a median position 

 in Testacella and Doris. 



The trypsin of the secretion of the " Mitteldarmdriise " converts 

 the proteids of the foodstuffs into peptones, so that they can be 

 absorbed by the enteric tract ; the amylolytic ferment converts 

 starch into sugar, and so renders it into a condition necessary 

 for absorption. This absorption probably goes on throughout the 

 whole length of the crop, stomach, and intestine, and more especially 

 by the epithelium covering certain conical processes of the mucosa 

 layer of those portions of the enteric tract. But the most interest- 

 ing fact in the physiology of the Mollusca is that a substance termed 

 glycogen has been found in most of their tissues, and especially 

 in the substance of this digestive gland, the " Mitteldarmdriise.^ 

 This glycogen (C 6 Hi O 5 ), as can be seen from its formula, is an 

 isomer of starch and dextrin. It is a white amorphous substance, 

 insoluble in alcohol and ether, and its aqueous solution has a 

 strong dextro-rotatory influence on polarised light. Its presence 

 in the tissues may be readily detected provided the animal has 

 been recently killed by testing with iodurretted potassium, with 

 which it gives a wine-red colour, disappearing on heating, reappear- 

 ing on cooling. In the Vertebrates, it has been detected in the 

 liver (i '5 4 per cent.), muscles, villi of the chorion, embryonal 

 tissues, placenta, and the white corpuscles of the blood. In the 

 Mollusca, Hammersten has demonstrated that in the liver of 

 Helix pomatia it is present to the extent of 1*75 per cent., and 

 that in hibernating animals of the same species, and in the same 

 organ, it is decreased to the amount of 0*429 per cent. ; Barfurtli 

 has found it in most of the tissues of Arion, Limax, Helix and 

 Cyclostoma ; while Blundstone has discovered it in the mesentery 

 of Helix. 



Whence comes this glycogen, and whither is it bound ? We 

 may get a suggestion or two regarding these questions which 

 we naturally ask, if we will call to our mind what is known of the 

 physiology of the Vertebrates in this particular, and compare it 

 with what has been demonstrated as existing in the Mollusca. 



