7 
molluses. Then the sand or gravel moves, and crabs appear. The 
shrimps, crabs, and hermit crabs run off with the smaller morsels; 
but the molluses 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 sup- 
posed 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 ma- 
terial 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 active little things, and are 
usually so different from their parents as to be unrecognisable. 
Some swim, or frisk about, and travel even long distances in search 
of suitable quarters to settle in. Others float on the surface, and 
are driven where the winds and currents list. Some, like mussels, are 
distributed all over the world, others again are found, perhaps, 
on one rock, or on one small sandbank in a large district. Many 
shells are rare, because we do not know where to look for them; 
but if we know and can find their food, we will find the shellfish 
not far away. Some change their shape so much that, as they 
age, they have to dissolve all the partitions made in their youth 
in the shell. The eggs of some are scattered on the surface of the 
water, while the eggs of others are hatched by the mother before 
being turned adrift. 
