CHAPTER XXXV
CRUSTACEANS
We now come to a very important class of animals, which includes the crabs, the lobsters, the shrimps, and the prawns. These creatures, together with the mollusks, are often called shell-fish, although the "shell" of a crab is not in the least like the shell of an oyster, for example, or like that of a whelk, or a snail. It is only a sort of crust upon the skin, made chiefly of carbonate of lime. That is why these animals are called crustaceans; and instead of growing, like true shells, this coat never increases in size at all.
But crabs and lobsters grow? Yes: but not as other animals do, a little every day. They only grow, as a rule, once a year; and they get a whole twelvemonth's growth into about two days!
When, in warm weather, the proper time approaches, they hide away in some crevice among the rocks, where none of their enemies are likely to find them. This is because they are going to throw off their so-called shells; and they know that when these are gone they will be deprived of their natural armor, and of their weapons too, and so will be quite at the mercy even of foes much smaller than themselves. Then a very strange thing happens. Part of their flesh actually turns to water! Sometimes, if you happen to take up a crab in a fish-market, and shake it, you will hear water swishing about inside it. This is a "watery" crab, and is not good to eat; for it was just about to change its "shell" when it was caught. A good deal of its flesh has actually turned to water.
Now this always happens a few days before the "shell" is thrown off; and the animal wriggles and twists about inside it, in order to loosen the attachments which bind it to its body. It also rubs its feelers against its legs, and its legs against one another, in order to loosen their hard coverings in the same way. This goes on, perhaps, for three or four days. Then, suddenly, the "shell" splits across, and the animal, with a tremendous effort, springs right out of it, while the "shell" closes up again, and looks just as it did before. One might really think that there were two crabs instead of only one.
For some little time the animal now lies perfectly still. It is exhausted by its efforts, and its muscles are so cramped that they feel quite hard to the touch. This cramp soon passes off, however; and then at once the animal begins to grow. It grows very fast. Indeed, you can almost see it grow, for a whole year's increase in size has to take place in about forty-eight hours. Then a fresh crust is gradually formed upon the skin, and two or three days later the animal is once more clad in a coat of mail, and is ready to leave its retreat and face its enemies. For a whole twelvemonth after this it grows no bigger. But at the end of that time the process is repeated, and so on, year after year, until at last the animal reaches its full size.
Forms of Crustaceans
The bodies of the crustacean animals are made up of a number of rings, or segments, like those of the insects. But there are always twenty of these rings, instead of thirteen; six forming the head, while there are eight in the thorax and six in the hind body.