ion.

The appetites of molluscs verge on the voracious. Break up a few
cockles, or other shellfish, and place them in shallow water on a calm
day, and watch the result. If in the vicinity of rocks, and during a
rising tide, all the better. First come the wary little shrimps to the
feast. Some are creeping cautiously, and some are jumping and racing, as
if afraid of not being in time. Then the carnivorous shellfish approach
from all directions, foremost amongst them being the different species
of Cominella. While they are lumbering along, shells appear to be
actually running; but a close inspection shows that these contain active
little hermit crabs, whose tender tails, having no hard covering of
their own, are snugly stowed in the empty shells of defunct molluscs.
Then the sand or gravel moves, and crabs appear. The shrimps, crabs, and
hermit crabs run off with the smaller morsels; but the molluscs gather
round the remnants and pull and haul and roll over one another until the
feast is ended, when some, being satiated, contentedly burrow into the
sand; while others, with their appetites only sharpened, will wander
away in search of fresh prey.

In many shells, such as the Triton, or Lotorium as it is now called
(Plate III.), every increase in growth can be traced in the thick lip
formed by the animal when it has increased the size of its shell. Others
again, such as the Struthiolaria (Plate IV., Fig. 4), only form a lip
when their full size has been attained, and by this the difference
between an old and young Struthiolaria can at a glance be seen. Others
form a lip at each growth, and then dissolve the lip before starting
again. Vertebrate fish are supposed to grow, and increase in size, till
the day of their death, but shellfish do not do this. The shell becomes
stronger and thicker with age, the animal having the ability to add
layer after layer of nacreous, or pearly deposit, on the inside of the
shell; and as the animal shrivels and lessens in size the thickness of
the shell increases. And some, when they become too large, have power to
dissolve the partitions in the shell, and deposit the material on the
outside of the shell.

The time it takes a shellfish to grow to its full size varies a great
deal. Oysters take about five years; but the giant Tridacna, the largest
bivalve in the world, has been found so enclosed in the slow-growing
coral that it could hardly open its valves.

The young of most shellfish are