182 



FRESHWATER FAUNA 



great part of the Tertiary period, when it flourished in 

 numerous extensive lakes and inland seas ; and the 

 species which is found in our islands, Dreissena poly- 

 morpha (Fig. 54), is said to be a recent immigrant, 

 having made its appearance in the Thames and other 

 rivers since the year 1824.* But its introduction is due to 

 human agency ; the mussel is provided with a byssus by 

 which it attaches itself to floating objects, ships among 

 Others, and has thus been carried not only up the 



Sm 



FIQ. 54. The Free-swimming Larva of Dreissena polymorpha, Pallas. 

 Figure on the left, lateral view; on the right, as seen from below. 

 raw. Mouth, ma. Stomach, d. Intestine, a. Anus. s. Sense 

 organ, sm. Shell muscle, vel. Velum, bearing locomotive cilia. 

 Magnified considerably. 



Thames, but the Rhine, the Danube, and other rivers. 

 Once introduced, its floating larvae ensure its distribution 

 over all that part of the river which lies below the 

 highest point reached by the adult. 



The freshwater univalves are Limnaea (the pond snail) 

 (Fig. 56), Ancylus (the freshwater limpet) (Fig. 75), 

 Planorbis and Valvata (Fig. 69), which belong to the 

 Euthyneura, and Paludina (Fig. 55), Bithynia, and 



* On this point see Scharff, "History of the European Fauna," 

 p. 27. 



