THE FRESH- WATER CLAM 227 



ern states we have certain large water snails which are allied 

 to species found on the coast of the Gulf of Mexico. The 

 Unios find their nearest marine allies among the mussels of the 

 sea. In fact in the group of mussels the attempt to accjuire 

 a fluviatile life has been made several times independently. 

 In Eurcpe a mussel, very similar to that of the seacoast, has 

 entered the rivers of Russia and, by the aid of connecting 

 canals, those of Germany (Dreisenia). Among the fishes there 

 are those that pass from the sea to rivers regularly once a 

 year during the spawning season. Others live in estuaries, 

 but travel up and down to a limited extent. In view of the 

 migration of fishes to-daj^ it is not difficult to see how some of 

 them, although of marine ancestry, came to live permanently 

 in the rivers. 



Of all the varied animals of fresh water perhaps the most 

 conspicuous, in our Central states, are the clams. These we may 

 now consider in detail. The fresh-water clams belong to the 

 group of lameUibranch nwllusks, characterized by the absence 

 of a distinct head, by the possession of leaf-like gills on the 

 sides of the bod_y, and bj^ the presence of a limy shell composed 

 of a right and a left valve. Economically this group is the 

 most important to man of all JNIoUusca. It includes both 

 marine and fresh-water species. About six thousand species 

 of living lamellibranchs and over ten thousand fossil species 

 are recognized. They all fall into two principal subdivisions. 

 The first (Siphonata) includes species which possess a siphon 

 and have the mantle edges grown together, while the second 

 (Asiphonata) has no siphon and has the mantle lobes for 

 the most part wholly separated. Consequently the first group 

 can live in the mud, drawing pure water from the surface, 

 whereas the second cannot do so. 



