ANCYLUS. 119 



This is in some respects an anomalous genus, although 

 undoubtedly related to Limncea. Menke and other con- 

 chologists have proposed to separate it from the Limnceidce 

 and to make it a distinct family by itself. The resem- 

 blance of its shell to the marine Limpet, or Patella, has 

 caused this to receive the not inappropriate name of the 

 " freshwater Limpet/^ — showing that the sea and land 

 have their respective representatives or analogues in the 

 system of Nature. It was for a long time supposed, 

 even by the great Cuvier, that the Ancyli were branchi- 

 ferous ; but it has now been satisfactorily ascertained, 

 by the careful investigations of Mr. Berkeley and other 

 able physiologists, that they are truly pulmoniferous, 

 although they are also capable (like other aquatic Pul- 

 monobranchs) of extracting air from the water for the 

 purpose of respiration. They inhabit both rapid and still 

 waters, attached to stones and the leaves of plants. They 

 are not inactive in their habits, but have never been 

 observed in a floating position. One of our native species 

 [A. fluviatilis) is nearly as amphibious as the Limmjea 

 peregra and L. truncatula ; and it may often be seen on 

 rocks at the side of waterfalls, having no other moisture 

 than the spray which occasionally falls on it. When it 

 crawls, only the tips of its tentacles, and sometimes the 

 front edge of its mantle, are visible. The only two 

 British species of Ancylus are apt to be infested with a 

 number of quasi-parasitic worms, as is also the case with 

 many kinds of LimncRa. The food of the Ancyli consists 

 of freshwater Algse or Confervse, as well as of decayed 

 vegetable matter. They are said also to eat or swallow 

 a certain quantity of very fine gravel or sand, apparently 

 to assist their digestion, which is very slow. They can 

 live a long time without any nourishment. 



