16 CONCHOLOGIA CESTRICA. 
characters, they possess a very complicated and excitable 
Muscular System, many ofthe muscles being united tothe 
skin. The ventral surface is covered with a thick muscu- 
lar layer forming a long disc, which is termed the foot. 
The contraction of these fibres produce wrinkles, which 
succeed each other from behind forward, thereby enabling 
the animal to glide over solid surfaces, or on the water. 
In those possessing a turbinated shell, a large muscle 
arises from the columella, and after dividing, is spread 
over the sides of the body to be inserted into the foot ; 
an arrangement which enables them rapidly to retract 
the body within the shell. Numerous other muscles 
within the body serve different uses in their economy. 
The Nervous System, is composed of several gangli- 
onic masses, connected by nerve filaments. The largest 
one is placed on the cesophagus, and is the functional 
representative of the brain. 
The Circulation, is carried on by a heart consisting 
of an auricle, and ventricle, usually placed on the right 
side of the body, at the base of the branchia, in the 
aquatic; or in the pulmonary cavity of the terrestrial 
species. The blood is usually bluish-white; in the 
genus Planorbis it is red. 
The Organs of Respiration, are either pulmonary, or 
branchial, and are for the most part lodged in a cavity 
on the right side of the body, just within the last whorl 
of the shell, the entrance to which is valvular, and may 
be seen opening and shutting at the will of the animal. 
The Vent, is usually found just anterior to the pul- 
monary orifice. A third opening, near the base of the 
right tentacle, leads to the generative organs. 
The Tentacles, (mostly four,) are either cylindrical, 
tubular, and retractile by inversion; or triangular, solid, 
conical, and contractile, with the head beneath the 
mantle. 
