100 GASTEROPODA. LIMNADZ. 
All the Limnade inhabit fresh-water; they respire air, and 
eat vegetable matter. | 
They differ from the Helicide in the number and structure 
of their tentacula, and position of the eyes; as well as in their 
internal structure, with regard to the position of the sexual 
organs. 
The head is divided from the foot by a strong transverse 
groove. The mouth is fleshy and retractile; and when with- 
drawn, the aperture is transverse. The organs of mastication 
consist of a horny palate, and two small jaws, between which 
an elastic and transversely grooved tongue is capable of being 
exserted. 
The cesophagus is rather elongated, and is more or less 
abruptly dilated before it enters the stomach, which is a true 
gizzard. On each side of the cesophagus are situated the sali- 
vary glands, which are composed of many lobes. The gizzard 
is short in most of the genera. The intestine, after leaving the 
stomach, receives the biliary ducts by one or two branches, and 
then continues of an uniform diameter, until it terminates in 
the anus, which is lateral. 
The liver occupies, at least, the last whorl but one of the shell. 
The ovarium forms the upper part of the spire, and is more 
or less enveloped by the upper or posterior extremity of the 
liver. The oviduct is generally very flexuose, and terminates 
laterally. 
The animal is fixed to its shell by the hinder part of its foot, 
which is strongly muscular. 
The nervous system, although probably essentially similar, 
is very different at first sight from that of Helicidee. It con- 
sists of a larger central portion (through which, at least in all 
the turrited genera, passes a very slender muscle from the foot, 
whose office seems to be that of assisting the retraction of the 
mouth), and is situated behind the middle of the cesophagus. 
It receives two nerves from a pair of ganglia, placed anteriorly, 
and which are either confluent, or distant and connected by a 
commissure. These two ganglia receive nerves from the parts 
which surround the mouth, but I have not succeeded in tracing 
any one of them to its origin. I have counted four nerves on 
