22 THE LIFE OF THE MOLLUSCA 
a trap-door (Plate VIII., Fig. 2), the “ operculum ”; 
when extended, this lies on the animal’s back, and 
in many cases forms a sort of pillion on which the 
shell rests (Plate II., Fig.1, and Plate VIII., Fig. 1). 
It consists of a horny layer, sometimes strengthened 
by the addition of shelly matter, which differs in 
structure from that of the shell itself. Its inner 
side is marked by the scar of the muscle which is 
attached to it, while outside it exhibits lines of 
growth. The operculum, various forms of which 
are shown on Plate VIII., Figs. 4-13, grows by 
additions made to the original point of beginning, 
the “nucleus,” either all round, concentrically; or 
on one side, so that the nucleus remains at the end 
or at one side; or the growth may result in a spiral. 
This spiral is sinistral in dextral, dextral in sinistral 
shells. Nearly all the Gastropoda are furnished 
with an operculum in the young stage within the 
egg, and though some discard it on hatching out, 
the greater number retain it throughout life. Many 
of the spiral and concentric forms fit the mouth of 
the shell accurately, others only partly close the 
aperture, in yet others it becomes rudimentary, while 
in certain specialized forms, such as Stvombus and 
its allies, it is converted into a sort of claw at the 
end of the elongated foot, and is used to assist the 
animal in progression (Plate VIII., Fig. 3). 
A more peculiar means for blocking the entrance of 
the shell when the animal retires into it is possessed 
