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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 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.
