232 
LIMNJSAD^. 
3. The animal has its three orifices on the left 
side of the neck, a character which distinguishes it 
from all the other dextral LimnceadcE. But this is 
only a displacement of the extremities of these 
organs ; for the organs themselves are placed on the 
right side of the body, as in all the other dextral 
LimncRadce. 
4. Consequently, he adds, the animal of Planorhis 
is essentially dextral, like the shell. 
5. The sinistral position of the orifices of the 
Flanorbis is the same exception in the Pneumobran- 
chous Mollusca, as the sinistral position of the gills 
of certain Ptenobranchous Mollusca is amongst the 
other genera of the order. 
Mr. Benson observes : The animal is considered 
as sinistral ; but if the shell be viewed as such prac- 
tically, and placed with the side which would in a 
sinistral shell be accounted the apex, it will be 
found that the animal is on its back, and that it will 
have to twist its body half round, in order to gain 
the ground with its foot ; and that, in order to creep 
with any ease, it must reverse the position of the 
shell. This is more especially observable in the 
flatter and more oblique-mouthed species.” Mr. 
Benson considers that face as containing the apex, 
which is contiguous to the back of the animal. This 
side may invariably be known in Planorhis by the 
greater projection of the lip in that part, by the 
deeper depression of the central umbilicus, and by 
the more considerable involution of the whorls, oc- 
casioning the greater depth of the suture. {Journ. 
Asiat Soc, Beng, 1836, p. 744.) 
