FRESHWATER MOLLUSCS 
The molluscs of interest to the aquarist belong so two classes. 
They may be described as animals devoid of bony structure and joints, with 
soft, thick and tough tunics or mantles, fleshy bodies and calcarious shells 
of one or two valves; the Gasteropods or Univalves and the Lamelli- 
branchia or Bivalves. ‘The first of these comprise the snails, limpets and 
slugs, and the second the mussels and oysters, They have simple diges- 
tive systems consisting of a mouth, a canal, digestive glands and anus; a 
contractile heart of several cavities with but few blood vessels, the blood 
being forced directly into the organs and through the spaces between them. 
The breathing structures are either comb-gills or simple air breathing 
chambers serving as lungs. The nervous system consist of special sense 
organs and ganglia of nerve substance located at different parts of the 
organism. 
Univatves. Most of the univalves have a single shell, but with 
some this is rudimentary, in others reduced to a few calcarious grandules 
beneath the mantle, though these latter are mostly land and marine forms. 
The shells of freshwater snails vary in form and 
may be flat-coiled, spiral, oval-oblong, elongate or 
earshaped, varying also from a length of two inches 
and over to microscopic sizes. Some havea horny or 
calcarious lid or operculum attached to the foot, 
whereby the aperture is closed when the snail has 
retired into it. Fig. 140. This is usually marked 
with curved striations about a central nucleus, the 

original operculum of the young snail. A mass of 
Freshwater snail. 
muscular tissue forms the foot, constituting the organ A. Apex. 
: ; B. Whorls 
of motion, and movement consist of its contraction Geos 
and expansion from the rear to the front. A part Denes 
; 5 f E. Body whorl. 
of the foot and the digestive system are enclosed in F. Periphery. 
: x lt G. Inner lip. 
the shell. The head is distinct and usually has FH, Guter ip, 
I. Operculum. 
two, sometimes four, tentacles serving as organs of 
touch and possibly of hearing. The eyes are distinct and may be devel- 
oped at the ends of a second pair of tentacles or upon longer or shorter 
pedicels, between, to the side of, or under the tentacles. 
In the aquatic species respiration is by gills in the water breathers, or 
by an air-chamber or rudamentary lung in the air breathers, the entrance 
to the breathing organs being near the mantle. 
The shell is formed by an excretion of carbonate of lime and some 
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