FRESHWATER MOLLUSCS 



The molluscs of interest to the aquariist 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. 



Univalves. 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 have a 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 

 muscular tissue forms the foot, constituting the organ 

 of motion, and movement consist of its contraction 

 and expansion from the rear to the front. A part 

 of the foot and the digestive system are enclosed in 

 the shell. The head is distinct and usually has 

 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 



FIG. 140. Outline of a 

 Freshwater snail. 



A. Apex. 



B. Whorls. 



C. Suture. 



D. Spire. 



E. Body whorl. 



F. Periphery. 



G. Inner lip. 

 H. Outer lip. 

 I. Operculum. 



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