Chap. XV.] THE SNAIL. 295 



into sugar. Beyond the crop the diameter of the canal narrows, 

 but soon expands again to form the stomach (si). Here the 

 digestive tube makes a sharp turn and proceeds as the intestine 

 (int.), which makes an S-shaped loop through the tissue of the 

 digestive gland (d. gl.), and then passes into the rectum (r.). The 

 large digestive gland is divided into two lobes, one of which 

 lies in the apex of the shell. This lobe pours its secretion into 

 the stomach by a special duct. The ducts of the three divisions 

 of the left lobe (d. d. gl.) unite to form a common duct. Three 

 kinds of cells are found in the digestive gland : (1) ferment cells, 

 which blacken rapidly with one per cent, osmic acid; (2) hepatic 

 cells, which contain spherical globules soluble in ether, and 

 excrete small vesicles which have yellowish contents, and are 

 evacuated with the faeces ; and (3) calciferous cells, containing 

 granules of carbonate or phosphate of lime, which are said to be 

 utilised in the formation of the hybernaculum. 



Some of the cylindrical epithelium cells lining the buccal cavity 

 are ciliated. A section of the oesophagus gives the following 

 layers: (1) an internal lining cuticle; (2) an epithelial layer; 

 (3) a lacunar mucosa layer; (4) a muscular layer, with (a) cir- 

 cular and (b) longitudinal fibres; (6) an external peritoneal 

 layer. The epithelium which lines certain depressions in the 

 stomach has been regarded as glandular, that covering certain 

 conical processes of the mucosa layer as absorptive ; and from 

 the distribution of the glandular and absorptive organs it would 

 seem that, with the exception of the buccal portion, which is 

 used for collecting food, and the terminal portion for expelling 

 faeces, no separate functions are assignable to the other divisions 

 crop, stomach, intestine of the alimentary canal. The fer- 

 ment secreted in the digestive gland would seem to be capable of 

 converting starch into sugar, and of digesting proteids. 



The Heart and Circulation. The heart (Fig. 85, B.) has a 

 single auricle (au.) and single ventricle (v.), between which there 

 is a valvular orifice. It lies in a pericardial chamber, which is in 

 communication with the renal organ. From the ventricle there 

 passes forward a single aorta. This runs back, passes dorsal to 



