5 LAND AND FRESH-WATER MOLLUSKS. 
long gullet (fig. 6, b), an intestinal canal, folded 
in the substance of the liver, bent to the right, 
and terminating close to the margin of the 
mantle in about the middle line of the body. 
The liver occupies a very large part of the 
abdomen. 
The Circulatory System is somewhat more 
perfect than in the bivalves. 
The Respiration is aquatic; the organ is a 
plume-like gill, formed of a series of triangular 
plates attached to the left side of the branchial 
cavity. The water passes into this chamber 
through a respiratory siphon formed by the 
folding of a small lappet of the neck. 
The Nervous System is much the same as in 
the bivalves. The eyes are two in number, 
placed on short stalks attached to the conical 
tentacles. The organs of hearing are situated 
near the base of the tentacles, and are sacs 
containing an otolith suspended in a fluid. The 
sense of smell is possessed by Gasteropods, and 
that of touch in a high degree; the tentacles 
are endowed with great sensitiveness. 
Reproduction. — The sexes are distinct in 
Paludina ; the eggs are retained within the 
oviduct of the parent until they are hatched, 
and the young are not excluded until they 
have attained a considerable growth. Palu- 
dina is therefore ovoviviparous. The young 
