PLANORBIS. 35 



I. PLANORBIS. 



II. Physa. 



III. LlMN^A. 



IV. Ancylus. 



GENUS L— PLANORBIS* GUETTARD. 



Body long, capable of being entirely contained within the 

 shell ; tentacles very long, slender ; respiratory orifice on the 

 left side ; foot oval, narrow, short, rounded in front and behind, 

 and attached to the body by a long, slender stalk ; jaw usually 

 single and slightly arched ; liftgual ribbon with straight trans- 

 verse rows, central tooth usually two-pointed, lateral teeth three- 

 pointed. 



Shell quoit-shaped or flattish ; spire dextral ; umbilicus more 

 or less distinct. 



The Planorbes inhabit sluggish streams and stag- 

 nant water ; they have the power of crawling and 

 floating, and frequently remain in a reversed position 

 on the under surface of the water. When the ditches 

 and marshes which they inhabit are dry some species 

 shut themselves up within their shell, by forming an 

 epiphragm round its aperture, till the return of rain. 

 They are vegetable feeders. When irritated they 

 instantly retreat into their shell, at the same time 

 often emitting a reddish coloured liquid, which is 

 secreted by a gland at the sides of the neck. 



The most remarkable characteristic of the Planorbes 

 is that while their respiratory, anal, and generative 

 orifices are placed on the left side, their shell is 

 dextral ; many authors have on this account regarded 

 the shell as sinistral, but Linne, Miiller and other 



Flat coil. 



D 2 



