35 ° 
MOLLUSCS. 
Pond-Snails, etc. 
EMBRYO OF RIVER-LIMPET (Ancyllls). 
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 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. 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 ( Vclletia ). 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. truncatulci) 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 stagnalis ; 
b, Ancylus fluviatilis ; c, Succinea putris. (Greatly 
magnified.) 
