THE SNAIL TRIBE 



3319 



Pond 



Snails, etc. 



rather solid, of different forms, but usually with narrow apertures, 



.. s> more or less contracted by teeth. The internal septa between the 

 Auricuhdae 



whorls of the spire are often absorbed or dissolved, excepting that 



between the last and penultimate volutions. Some species of the typical Auricula 

 have large heavy shells, and are met with in mud banks and in swamps in the Indian 

 Archipelago. Those belonging to Pythia have oval, compressed shells, with toothed 

 apertures, and occur in great numbers in most places in woods near the sea, are 

 wholly terrestrial in their habits, and feed on decayed vegetation. One member of 

 this family (Carychium minimum} is found in Great Britain, and on the Continent. 

 It is an extremely small shell, less than the tenth of an inch in length, transparent, 

 glossy, having three teeth-like projections within the aperture. It hides away at 

 the roots of grass, among moss, dead leaves, or under stones or dead wood, in damp 

 situations. 



The three families Limnceidce, Physidce, and Chilinidce form a 

 second group of the Pulmonates with sessile eyes, all being inhabit- 

 ants of fresh water, but rising occasionally to the surface to renew the 

 supply of air. They are mostly herbivorous, but 

 some kinds of Limncea and Physa are said to become 

 carnivorous occasionally. In the first family the 

 members of the genus Ancylus are popularly known 

 as fresh-water limpets, on account of the resemblance 

 in form of their shells to the true limpets. They all 

 have small thin shells, with the apex somewhat pos- 

 terior, but generally inclining a little to the right or 

 left. They are found on all four continents, as well 

 as in Australia, New Zealand, the West Indies, and 



other islands; two species being British. A. fluviatilis occurs generally on stones, 

 but occasionally on plants in shallow streams, and running brooks, whereas A. lacus- 

 tris invariably adheres to the stems and leaves of plants in ponds, lakes, and canals. 

 On account of this difference of habit, and also for various anatomical reasons, the 

 latter species has been placed in a separate section ( Velletia}. The illustration rep- 

 resents the embryonic stage of this species just prior to its quitting the ovum. 



The fresh-water snails belonging to the genus Limncea have thin horny shells, 

 with more or less sharp spires, and are usually dextral, but certain forms occurring 



in the Sandwich islands, Australia, and 

 probably other neighboring localities, 

 are constantly sinistral, so that it is 

 impossible to separate them from the 

 genus Physa by the shells only. An 

 examination of the animals, however, 

 at once determines their proper posi- 

 tion. In Limncea the two tentacles are 

 compressed and triangular, with the 

 ROW OF TEETH OF RADULA oF-a. Limnaa stagnate ; eyes at their inner base. In /%!/.* they 



*. Ancylus fluviatilis; c. Succinea putris. (Greatly 



magnified.) are cylindrical. The jaw and radula 



EMBRYO OF RIVER UMPBT 

 (Ancylus). 





