Because the stomach is formed on a bony apparatus, in short, a species of skeleton ; and does not therefore collapse when empty. Hence the policy of choosing crabs and lobsters by their weight.
Because to certain parts of the bony structure of the stomach, round its aperture communicating with the small intestines, (or the pylorus) the teeth are affixed. They are extremely hard, and serrated, or jagged, and as they surround the tube near the pylorus, nothing can pass that has not been duly prepared. These bones and teeth (the latter three in number) are moved by peculiar muscles, and in the craw-fish are known to be annually reproduced.
Because they may thus completely conceal their form, and secure themselves from the detection of their enemies.
Because they furnish the principal materials from which the new shell is hardened. Some are naturally red, whilst others remain black, even when boiled ; and some reach the age of twenty years.