14 
LAND AND ERESH-WATER MOLLUSKS, 
of the mantle are united near the posterior margin 
and pierced by two orifices; the upper one is 
called the anal opening (fig. l,v), the lower one the 
branchial opening , b. 
The digestive organs .—The mouth (o) is a hori¬ 
zontal opening situated anteriorly just above the 
foot, between two pairs of soft flattened lips (t); 
Dorsal margin. 
Ventral margin. 
Fig. 1. 
it is rather difficult to find, but may be discovered 
by tracing the lips to their insertions, between 
which the mouth is situated. The mouth opens 
immediately into a stomach, which contains a 
cylindrical jelly-like body, termed the crystalline 
style , whose function is not known. The intestinal 
canal is a narrow tube wound around a large liver, 
and after passing through the heart, is continued 
along the back to the vent (v), which is situated 
near the anal orifice : the excremental matters 
are carried away by the water which had passed 
over the gills. 
