SHELL GALLERY. 
The oper- 
culum. 
The 
breathing- 
organs. 
as in the Snail or the Slug, and so give rise to an air-chamber, 
which, when its walls are richly supplied with blood, serves as a 
lung. The ventral surface of Molluscs is produced into the so- 
called " foot," which may be very variously modified. The foot 
may be more or less hatchet-shaped, or curved and capable of serving 
as a leaping-organ, or sole-shaped and adapted for creeping ; its 
margins may be produced into elongated processes, as the so-called 
arms of the Octopus, eight in number and provided with suckers, 
or of the Nautilus, where the arms are much more numerous, but 
shorter and without suckers. In the Cephalopods, also, another 
part of the foot may fold over from either side and form a median 
funnel, through which the water of respiration is driven outwards, 
causing the animal to move in the opposite direction — this part of 
the foot having, therefore, still the function of an organ of loco- 
motion. By means of their muscular foot the Solenidce, or Razor- 
shells, burrow in the sand, the Pond-Snails (Lim?iceidce) crawl on 
aquatic plants and swim reversed on the surface of the water, the 
Limpet clings to the rock, and the Cockles and Trigonias take 
surprising leaps. 
Upon the upper surface of the foot, in many Gastropods, a flat 
hard structure termed the operculum is situated, which, when the 
animal is retracted, partly or entirely closes the aperture of the 
shell. In some cases, as in the Turbos, it is very strong and 
of a stony nature, but in most instances it is horny. It is 
differently constructed in distinct families : it may be annular 
and multispiral, annular and paucispiral, subannular and ovate, or 
subannular and unguiculate. In the Nerites it is shelly, somewhat 
semicircular, closes the aperture of the shell, and is furnished with 
a stout projection on the straight edge, fitting like a hinge under 
the inner lip of the shell. A series of opercula is exhibited in side 
table-case C. 
Thread-like processes on either side of the body, the so-called 
gill-filaments, often unite with those in front of and behind them, 
and so give rise to plates ; these, when well developed, are best 
seen in the division to which the Oyster and the Mussel belong, 
and which, therefore, has been called the division of the plate-gilled 
Molluscs, or Lamellibranchia. Where the body is coiled or twisted 
on itself, as so often happens, the gills of one side may be altogether 
lost. Sometimes, as in Phyllirho'e\ when the body is small and its 
wall thin, the gills (ctenidia) disappear altogether, and there is no 
special breathing-organ ; in others the loss of the gill is compensated 
