35 



MOLLUSCS. 



Pond-Snails, etc. 



EMBRYO OF RIVER-LIMPET (Ancylus). 



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 have all small thin shells, with the 

 apex somewhat posterior, but generally inclining 

 a little to the right or left. They are found on all 

 four continents, as wflll 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. lacustris 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 (Vdletia). The illustration represents 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 neighbouring 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 position. In 

 Limncea the two tentacles are com- 

 pressed and triangular, with the eyes 

 at their inner base. In Physa they 

 are cylindrical. The jaw and radula 

 are also different in the two genera. 

 The species of Limncea frequent 

 shallow and still waters, in most parts 

 of the globe, often swimming at the 

 surface of the water in an inverted 

 position. They are prolific and 

 gregarious, and the eggs are enclosed 



in transparent gelatinous capsules, and deposited in continuous series, and attached 

 to submerged stones as well as to the stems and leaves of aquatic plants. L. 

 stagnalis, which is common in ponds, marshes, and slow rivers, is the largest species, 

 and six other species occur in Britain. The other figured forms illustrate the 

 great variation which occurs in the relative length of the spire and aperture. In 

 one of these (L. truncatula) the liver-fluke, so destructive to sheep, passes. one stage 

 of its existence. 



The animals of the genus Planorbis are small, and have the vital organs on 

 the left side. The tentacles are slender as in Physa, but there are no lateral 

 mantle-lobes. The shells are all very similar, being sinistrally and spirally coiled 



ROW OF TEETH OF RADULA OF a, Limncea stagnates ; 

 b, Ancylus fluviatilis ; c, Succinea putris. (Greatly 

 magnified.) 



