GASTEROPODA. 
11 
1. Is developed on the embryo long before it is 
hatched. 
2. That it is placed on and covers a peculiar part 
of the body, which bears the same relation to it as 
the part of the body called the mantle, bears to the 
part usually called the shell of these animals ; and it 
is formed, and increases in size, by an opercular man- 
tle in the same way as the shells are. 
3. That the operculum is more or less conical, and 
is increased in size by the addition of new matter to 
the inner surface, and especially to the part of it near 
the margin, the new matter either forming more 
or less complete rings round the nucleus (or first- 
formed part) — when it is called annular^ and is homo- 
logous to the simply conical shells, as the Patella ; 
or the new matter is deposited almost entirely on 
one edge of the nucleus, when the operculum forms 
a more or less elongated cone, which when long is 
generally twisted round an imaginary axis (like a 
spiral shell), the broad part of the cone being next 
the edge of the opercular mantle which generates 
the new matter for enlarging its size, as the mouth 
of the shell is on the outer edge of the mantle of the 
univalve shell. 
4. That the operculum is attached to the animal 
by means of one or more muscles which, as in inequi- 
valved bivalved shells, pass from the larger valve, or 
shell, to the smaller one, or operculum. 
5. The operculum as it increases in size is gra- 
dually moved on the end of the muscle ; the many- 
whorled opercula of the trochi revolve as many 
times on the end of the muscle as the many-whorled 
