210 LAND AND FRESH-WATER MOLLUSKS. 
Planorbis (Coil Snails)o 
The shells of this genus are characterized as 
orbicular, flat, and coiled nearly in the same 
plane. The generic term signifies a flat coiL 
Planus , flat; orbis, coil or ball. 
The animals inhabiting these discoidal shells 
are very slender, elongated, and twisted in a 
flat coil; they possess two very long and slender 
tentacles, the eyes situated at their inner bases. 
The foot is small; the lingual dentition is very 
simple : the transverse rows extend straight 
across the lingual membrane : the central tooth 
is generally two-pointed, the laterals with three 
points. The jaw is single and slightly arched 
(except in P. corneus ), as in Physa , and in fact as 
in Pupa , Balea , Glausilia , and Oarychium. The 
shells of this genus exhibit great tendency to 
distortion. 
The species that compose this genus are 
numerous, inhabit slow running" streams, ponds,, 
and ditches, feeding on the aquatic plants y 
and are very sluggish in their movements. A 
peculiarity, possessed by all of the genus, may 
be readily observed by irritating the animal of 
P. corneus or P. marginatus , when a purplish 
liquid is emitted, which is not the blood, for the 
circulating fluid is colourless. The majority of 
