SYSTEMATIC 
ARRANGEMENT OF MOLLUSCS. 
CLASS XIII. 
MOLLUSCA. 
ANIMALS covered by a soft moist skin, mostly forming over the 
back a duplicature, free at the margin (mantle). Head more or less 
distinct, furnished with tentacles and often with two eyes. Shell 
calcareous, mostly univalve, in some multivalve, never bivalve; in 
a few internal, in some absent. Organs of circulation and respi- 
ration mostly distinct; heart always aortic. A nervous ring around 
the cesophagus ; nerves proceeding from ganglia, various in number, 
towards the peripheral parts of the body. 
Many aquatile, some terrestrial, almost all swimming or 
creeping. 
Orper I. Prteropoda. 
Molluses furnished at the anterior part on both sides with a 
natatory expansion or pinna, with head often little distinct, herma- 
phrodite, marine. 
Wing-footed. Cuvier first distinguished this division under this 
name in 1804. He characterised it by the absence of the foot or the 
ventral disc ; this part, however, appears to be not so entirely want- 
ing, though formed in a different fashion, especially laterally, and to 
be developed into the so-named wings or fins. Hence some writers, 
and amongst them SouLEYET, unite the Péeropoda with the Gastero- 
poda. At all events they have a closer alliance with these than 
with the Cephalopoda, and must, on account of the simplicity of 
structure, be placed below the Gasteropoda. 
