ORDER HETEROPODA. 49 
M. Reynaud has just brought an individual which had lost its 
shell, but where, it appears, traces of the membranes which 
attached it to the mantle may be perceived, notwithstanding 
which, no remains of muscles are visible. A similar shell is 
also found in the Mediterranean : its animal, however, has not 
yet been observed. 
FIFTH ORDER OF GASTEROPODA. 
HETEROPODA, Lam. 
THESE are distinguished from all the others by their foot, 
which, instead of forming a horizontal disk, is compressed 
into a vertical muscular lamina, which they use as a fin, and 
on the edge of which, in several species, is a dilatation form- 
ing a hollow cone, that represents the disk of the other orders. 
Their gills, formed of lobes like feathers, are situated on the 
hind part of the back, directed forward; and immediately in 
their rear are the heart and a small liver, with part of the 
viscera, and the internal organs of generation. ‘Their body, a 
gelatinous and transparent substance, lined with a muscular 
layer, is elongated, and usually terminated by a compressed 
tail. There is a muscular mass belonging to the mouth, and 
a tongue furnished with little hooks; the wsophagus is very 
long, their stomach thin; two prominent tubes on the right 
side of the visceral bundle afford a passage to the fceces, 
semen, and eggs. They usually swim on their backs, with 
the foot upwards. This mode of swimming having induced 
Peron to think that the natatory lamina was on the back, and 
VOL. XII. E 
