Then—again like the insects—crustaceans have feelers, or antennæ, upon their heads. You can see these very well indeed in a lobster or a shrimp. But instead of having one pair of these organs, as insects have, they always possess two pairs. And it is rather curious to find that at the base of the front pair there are two little organs which seem to be ears, specially formed for hearing in the water, while at the base of the second pair are two other little organs which seem to serve as a nose, specially made for smelling in the water.
And—once more like the insects—crustaceans have to pass through several different forms before they reach the perfect state. They are hatched in the first place from eggs, which the mother animal carries about with her for some little time firmly fastened to the hairs of the swimmerets, which we find under the hind part of her body. You will often find a shrimp with quite a large bunch of these eggs; and if you look at them carefully with a good strong magnifying-glass, you will see that they are all glued down to hairs.
Inside each of these eggs an odd little creature is formed, which is called the nauplius. Sometimes it is hatched while still in that state, and swims about through the water. But in almost all the higher crustaceans a change takes place before it leaves the egg, and it appears at last in the form of a zoëa.
This is a kind of crustacean caterpillar, and a very odd little creature it is. A great naturalist once described it as an animal "with goggle eyes, a hawk's beak, a scorpion's tail, a rhinoceros' horn, and a body fringed with legs, yet hardly bigger than a grain of sand!" Certainly it does not look in the least like the crab, or lobster, or shrimp into which it is going, by and by, to turn. And it swims in the oddest way possible, by turning endless somersaults in the water!
These zoëas are sometimes found in immense shoals, the surface of the sea being quite thick with them for miles. And they are useful little creatures, for they feed on the tiny scraps of decaying matter which are always floating about in the sea, just as tadpoles and gnat-grubs do in ponds, thus helping to keep the water pure. But a very great number of them are devoured by whales. For when whalebone-whales are hungry, they swim with open mouths through a shoal of these little creatures, and then strain them out of the water by means of the whalebone fringe which hangs down from the upper jaw.
After a time the zoëa throws off its skin and appears in quite a different form. It is now called a megalopa, or big-eyed creature, because it has very large eyes, which are usually set on foot-stalks, and project to quite a long distance from the sides of the head. And as the zoëa is a kind of crustacean caterpillar, so the megalopa is a kind of crustacean chrysalis. It generally has a long, slender body, made up of several joints. And it swims by flapping this to and fro in the water.
Crabs
First among the crustaceans come the crabs, of which there are a great many different kinds. They are distinguished by having the tail tucked under the body, and firmly soldered, so to speak, to the "shell" on either side.
You can find several kinds of these creatures by hunting among the rocks on the sea-shore when the tide is out. There is the common shore-crab, for example, which is green in color. It is generally to be found hiding under masses of growing seaweed. But sometimes you may see it prowling about in search of prey. It is wonderfully active, and will even pounce upon the sandhoppers as they go skipping about, just as a hunting-spider will pounce upon flies, seldom or never missing its aim. It will catch flies, too, leaping upon them when they settle, and shutting them up, as it were, in a kind of cage formed by its legs. Then it pokes one claw carefully into this cage, seizes the prisoners, pulls them to pieces, and pokes the fragments into its mouth.
Swimming about in the pools, too, you may often find a fiddler-crab, which is so called because its movements in the water rather remind one of a man who is playing the violin. You will find that its hind legs are very much flattened, and are fringed with stiff hairs, so that they may be used as oars. In fact, the animal rows itself through the water. Both these crabs, sad to say, are cannibals, and are always ready to attack and devour their own kind.