FAMILY LYMNiEACEA. 133 



Water is their natural element, but they can also live out of the 

 water. For animals dwelling in shallow ponds and ditches, which 

 are full or empty according to the amount of rainfall, and in 

 summer are liable to be dried up for weeks together, such an ar- 

 rangement of the breathing functions is necessary to existence. 

 The respiratory organ is a vascular sac, over which the blood flows 

 and is aerated in a network of minute vessels; and it is fitted 

 with branchial plates, or lamellae, for the respiration of water. 

 The British species are all contained in one family : — 



1. Iiymneeacea. Head produced into a short broad muzzle, 

 tentacles sometimes slender and bristle-like, sometimes 

 flatly triangular, with the eyes at the inner base. Shell ex- 

 tremely variable, sometimes flatly discoid, sometimes ovately 

 or fusiformly spiral, sometimes limpet-shaped. 



Family I. LYMNJEACEA. 



Head produced into a short broad muzzle, tentacles sometimes slender and 

 bristle-like, sometimes flatly triangular, with the eyes at their inner 

 base. Shell extremely variable, sometimes flatly discoid, sometimes 

 o-vately or fusiformly spiral, sometimes limpet-shaped. 



The Lymnmacea, or Pond Snails, are much less numerous over the 

 globe in species, with far less variation of typical form, than the 

 Land Snails ; and they are fewer and less varied in the Eastern than 

 in the Western Hemisphere. Yet they are comparatively numerous 

 in Britain, plentiful in individuals, and very generally diffused. 

 There are not many more species in all Europe than there are in 

 the British Isles. They have no particular centre of creation in any 

 part of their large area of distribution, which reaches from Greenland 

 and Siberia to the Himalayas ; and through all this wide range of 

 latitude there is Httle difference between the species of any genus 

 in form and colour. Below the Himalaya range to Tasmania, the 

 JjymiuBCB and Physce appear in new forms, illustrative of a number of 

 very interesting species ; but the principal area of development of all 

 our genera of freshwater mollusks is in Central and North America, 

 extending southwards to Chili, and northwards to Yancouver's 

 Island. Here the species are more abundant, and it will be seen 

 that they are distributed in more distinct assemblages of types. 



There are few groups of mollusks in which so variable a shell, 



