UNIVALVES. AZ 
in close contact with the ventral region of the 
body; from which circumstance the class has 
Fig. 5.—Paludina vivipara (Woodward). 
procured the name “ Gasteropoda,” or “ belly- 
footed.” This organ consists of a mass of inter- 
woven muscular fibres, and progression is due 
to the alternate action of a set of muscles on 
each side. 
The Head is prolonged into a muzzle, at the» 
extremity of which is the mouth; from the 
extension of the foot in front, the animal can 
only feed when at rest. The head bears two 
long and slender tentacles or horns with the 
stalks bearing the eyes attached to them on 
the outside. The tentacles are extended to the 
utmost, and are gently swayed to and fro when 
the snail walks: by means of them the animal 
feels its way, and is warned of danger. An ap- 
pendage may be observed on each side of the 
head arising from the tentacles ; that on the nght 
side is the largest. The operculum is situated 
on the hinder part of the foot. 
