No. 5. — 1849.] FRESHWATER MOLUSK3 



157 



Bithinia were detected. Many specimens were procured 

 and taken home, and other descriptions now given of their 

 habits and form are taken from examples which are living, 

 and have been under my close observation for several weeks. 



Planorbis Tennentii. 



The largest example found measures two-eighths of an 

 inch across the widest place. Whorles three, transparent 

 and colourless, showing the animal within ; when examined 

 through the microscope the substance of the shell may be 

 perceived to be laid on in successive layers. When the 

 molusk is alive it gives a spotted appearance to the shell ; 

 mouth inferior. 



Habitat, dead leaves upon tanks and pieces of wood, on 

 which they feed. 



The molusk is endowed with the power of swimming 

 along the surface of the water in the manner of the Lywma; 

 occasionally they reach the surface simply by detaching 

 themselves from the bottom, when they rise by the air they 

 have carried down with them on some previous occasion ; 

 but when the air is all exhausted by a long continuance 

 below the surface, they crawl up some aquatic plant till the 

 air is reached ; the foot is then thrown out on a plane with 

 the water, the long tentacles which are situated far back upon 

 the body are moved rapidly about as if seeking for some 

 support, the hold of the plant is gradually loosed and 

 brought to bear on the surface, and with two or three 

 sudden jerks the little voyager is fairly afloat ; its method 

 of progression is by suction ; the whole foot is on a level 

 with the surface of the water, which offers sufficient resis- 

 tance for it to propel itself along by alternate contraction 

 and expansion, the edges of the foot being frequently raised 

 above the surface. 



Unlike the common Planorbis Indicus, which carries its 

 shell erect, Planorbis Tennentii crawls with its shell almost 



