78 LIMN^ID^. 



Genus I. PLANOR'BIS^ Guettard. PL IV. f. 1, 2, 3. 



Body long, twisted in a flat coil : tentacles very long and 

 slender : foot short and narrow, attached to the upper part of 

 the body by a stalk, which is shaped like the lower half of an 

 hourglass. 



Shell quoit-shaped, or flat : ivJiorls cylindrical : spire dex- 

 tral, or turning from left to right, and visible on each side. 



This genus has some remarkable peculiarities. One 

 of them consists in the habit of the animal emitting its 

 purple- coloured blood, or a fluid like that which is se- 

 creted by the Aplysia, on being irritated, apparently as a 

 means of defence against its enemies. Another is, having 

 several of its vital organs placed on the left side of its 

 body, instead of on the right (as is the case with nearly all 

 the other Gasteropoda), wdiile the spire of its body and 

 shell is coiled the other way, viz. from left to right. And 

 a third peculiar feature consists in the form of its shell, 

 which is flat or concave on one or both of its sides, re- 

 sembling that of an Ammonite. The body of these mol- 

 lusks is too small for its shell ; and when crawling, the 

 animal leaves part of the shell empty, putting one in 

 mind of loose and ill-fitting clothes. O. F. Miiller, nearly 

 a century ago, seems to have satisfactorily settled the 

 question that was then raised as to whether the shells 

 were right- or left-handed ; but the discussion has lately 

 been renewed. The shell being viewed in its natural 

 position, there can be no doubt of its being dextral. 

 Some of the smaller species of Planorbis, inhabiting 

 marshes and very shallow water which are dried up in 

 summer, close the mouth of their shell with an epi- 

 phragm, or filmy covering, like that of some land-snails. 

 The animal then retires into the interior of its coil and 



* Flat-coil. 



