406 
THE OCEAN WORLD. 
is short, and very nearly round. The head is sufficiently distinct, and 
furnished with two very long filiform, contractile tentacles, having at 
their base, and on the inner side, a small organ, which looks like an 
egg. The mouth is armed in the upper part with cross-cutting teeth, 
and in the lower part with a tongue, bristling with a great number 
of hooked excrescences. 
In habits the Planorbis resemble the Limneans : they creep lib 6 
them on the surface of solid bodies, and swim in the water with the 
feet upwards and the shell down. They feed on similar substance^ 
and thoir eggs are collected in gelatinous masses also. They pass the 
winter in a state of torpor, buried in the mud of the rivers they 
inhabit. 
The principal species is Planorbis corneus (Fig. 201), which i s 
common in the rivers of England and France. 
Another group of molluscs, which occupy our fresh rivers and swi# 
with the shell down and feet up, is represented by Phy s<l 
castanea (Fig. 202). The genus Phym have an oval 
oblong, or nearly globular shell, very thin, smooth, a»d 
fragile, opening longitudinally, narrow above, with th 0 
right edge sharp ; the last turn of the spiral being larged 
of all. 
The animal appears to be intermediate in form betwe 0B 
Planorbis and Limnea : it is oval in form, and unrolls itself like tb e 
Limneans, but its tentacles, in place of being triangular and thi 0 b 
like the latter, are elongated and narrow, like those of Planorh 18 - 
These little inhabitants of fresh water swim with facility, the f ee ^ 
upwards, the shell below, like the Limneans, and they feed on veg e ' 
tables, like them. 
Fig. 202. 
Physa 
castanea 
(Lamarck). 
