WE have no word in English to indicate the varied crab-and-shrimp-like creatures of salt and fresh waters in the same way as "insect" designates the six-legged, usually winged, terrestrial creatures of many kinds—beetles, bees, bugs, two-winged flies, dragonflies, day-flies, and butterflies. They are all "insects." Naturalists call the aquatic shrimp-and-crab creatures "crustaceans." Perhaps "crab" might be used in a large sense to include them all, together with the true crabs, as the Germans use their word, "krebs." The shore-crab is the most familiar of all crustaceans, in the living, moving condition. Boiled lobsters, prawns, and shrimps are more generally familiar members of the class, but the "undressed" living crab is better known to every one who has been on the seashore than the live lobster, prawn, and shrimp. Londoners have been heard to express interest in the curious blue variety of lobster caught on the coast, not being aware that the hot bath which he takes before he, too, is "dressed," causes his blue armour to change its colour to a brilliant scarlet. Occasionally a regular ordinary lobster is caught in which this change has occurred during life in the sea—and there are some enormous deep-sea prawns of a pound in weight which when living have a splendid crimson colour. A large series of "crustaceans," carefully prepared so as to show their natural colours in life, is exhibited in the Natural History Museum in Cromwell Road.

A curious kind of prawn (by name Althea rubra), of fair size, is found under "the low-tide rocks" in the Channel Islands, which not only is of a deep crimson colour, but snaps his fingers at you—or rather one of his fingers—or claws—when you try to catch him, making a loud crack audible at ten yards distance. The common big prawn, if you see him in a large vessel of sea-water with the light shining through him, appears very brilliantly marked with coloured bands and spots—reddish-brown, blue, and yellow—which are displayed on a transparent, almost colourless surface. Of course, boiling turns him pale red. A common smaller species of prawn when boiled is often sold as "pink shrimps," and lately a deep-sea prawn—a third species—has come from the Norwegian coast into the London market. There are many kinds which are not abundant enough to become "marketable." Prawns are at once distinguished from the true "brown shrimp" by having the front end of the body drawn out into a sharp-toothed spine, which is absent in the shrimp. Besides the prawns (PalÆmon and Pandalus), the shrimp (Crangon), and the common lobster (Homarus), you may see in the London fish shops the large spiny lobster (Palinurus) called "langouste" by the French, and apparently preferred by them as a table delicacy to the common lobster, although it has no claws. It used to be called "craw-fish" or "sea craw-fish" in London; why, I am unable to say. The name was certainly bad, as it leads to confusion with the cray-fish, the fresh-water lobster of British and all European rivers (there are many other kinds of fresh-water lobsters in other parts of the world, as well as fresh-water prawns and crabs), whose English name is a curious corruption of the French one, "Écrevisse" (cray-vees, cray-fish). Another lobster of our markets is the little one known as the "Dublin prawn," which is common enough on the Scotch and Norwegian coasts, as well as that of Ireland. Naturalists distinguish it as Nephrops Norvegicus. The great edible crab completes the list of British marketable crustaceans, but in Paris I have eaten, as well as at Barcelona, a very large Mediterranean prawn, three times as big as our biggest Isle of Wight prawns, but by no means so good. It is called "Barcelona prawn" and "Langostino" ("PenÆus" by naturalists). In Madrid I have seen in the fish shops and eaten yet another crustacean—a very curious one—namely, a long-stalked rock-barnacle of the kind known to naturalists as Pollicipes.

That the barnacles—ship's barnacles (Fig. 10) and with them the little sea-acorns (Fig. 11), those terribly hard and sharp little white "pimples" which cover the rocks nearly everywhere just below high-tide mark, and have so cruelly lacerated the hands and shins of all of us who swim and have had to return to a rocky shore in a lively sea—should be included with crabs, lobsters, and shrimps as "crustaceans" must appear astonishing to every one who hears it for the first time. The extraordinarily ignorant, yet in their own estimation learned, fishermen of the Scottish coast will tell you with solemn assurance that the ubiquitous encrusting sea-acorns are the young of the limpet, whilst the creature living inside the shell of the long-stalked ship's barnacles has for ages been discoursed of by the learned as one of the marvels of the sea—nothing more or less than a young bird—the young, in fact, of a goose—the barnacle goose which, since it was thus proved to be a fish in origin, was allowed to be eaten by good Catholics on fast days! Two hundred years or more ago this story was discredited by serious naturalists, but the barnacles and sea-acorns were thought (even by the great Cuvier) to be of the nature of oysters, mussels, and clams (Molluscs), because of their possessing white hard shells in the form of "valves" and plates, which can open and shut like those of mussels. Their true history and nature were shown about eighty years ago by a great discoverer of new things concerning marine creatures, Dr. Vaughan Thompson, who was Army Medical Inspector at Cork, and studied these and other animals found in the waters of Queenstown Harbour.

The crab class, or Crustacea, have, like the insects, centipedes, spiders, and scorpions, a body built up of successive rings or segments. The earth-worms (as every one knows) and marine bristle-bearing worms also show this feature in the simplest and most obvious way. The vertebrates, with their series of vertebrÆ or backbone-pieces and the body muscles attached ring-wise to them, show the same condition. The marine worms have a soft skin and a pair of soft paddle-like legs upon each ring of the body, often to the number of a hundred such pairs. But the crab class and the classes called insects, centipedes, arachnids, and millipedes are remarkable for the hard, firm skin, or "cuticle," which is formed on the surface of their bodies and of their legs, which, as in the marine worms, are present—a pair to each body-ring or segment—often along the whole length of the body as in centipedes. This hard cuticle is impregnated with lime in the bigger members of the crab class, such as the lobster. It is not equally thick and hard all over the surface of the lobster, but is separated by narrow bands of thin, soft cuticle into a number of harder pieces, thus rendered capable of being bent or "flexed" on one another. Thus the body is jointed into a series of rings, and the legs are also divided each into several joints (as many as seven), which gives them flexibility and so usefulness of various kinds. The various joints are "worked" by powerful muscles, which are fixed internally to the cuticle and pass from one hard ring or segment, whether of body or of leg, to a neighbouring ring.

Every one knows the structure of a lobster's tail and of its legs, which can be readily examined in illustration of my statement, and the same structure can be seen in the leg of a beetle or a fly. Naturalists term all this series of creatures with hard-jointed cuticle, to which the muscles are attached, including the crab class, the insects, centipedes, spiders, and scorpions, "jointed-leg owners," or Arthropods. It is easy to appreciate this characteristic difference which separates the Arthropods from other animals. The sea-worms differ from them, in that they have soft cuticle, but stiffen and render their paddle-like legs firm by squeezing the liquid of the body into them in the same sort of way as the sea-anemones distend their tentacles with liquid, though in that case the liquid is sea-water taken in by the mouth. The Molluscs also distend their muscular lobe, or "foot" as it is called, by pressing the blood from the rest of the body into it, and so making it swell and become stiff, so that the muscles can work it; when not distended in that way it is flaccid. The Vertebrates (bony animals) and the star-fishes have again another and peculiar mechanism. Their muscles are attached to hard internal pieces, sometimes cartilaginous but often calcareous or bony, which are spoken of as "the internal skeleton." There are thus three distinct kinds of mechanism in animals for giving the necessary resisting surfaces, hinged or jointed to one another, and made to "play" one on the other by the alternate contraction and relaxation of the muscles attached to them.

The Arthropods differ among themselves in the number of body-rings, the enlargement or dwindling of certain rings, and the fusion of a larger or smaller number of the rings to form a composite head, or a jointless mid-body or hind-body. The successive legs are primarily and essentially like to one another, and each body-ring, with its pair of legs, is but a repetition of its fellows. At the same time, in the different classes included as "Arthropoda" a good deal of difference has been attained in the structure of the legs, and they have in each class a different form and character in successive regions of the body, distinctive of the class, and are sometimes, but not always, absent from many of the hinder rings. All these Arthropods agree in having a leg on each side immediately behind the mouth—belonging to a body-ring, which is fused with others to form the head—very specially shortened, of great strength and firmness, and shaped so as to be pulled by a powerful muscle attached to it, against its fellow of the opposite side, which is similarly pulled. These two stumpy legs form thus a powerful pair of nippers called "the mandibles." They are jaws, although they were in the ancestors of the Arthropods merely legs. These jaw-legs, or leg-jaws, are characteristic of all the crab class, as well as of the other Arthropods, but no bristle-worm or other animal has them. The jaws of marine worms are of a totally different nature. So are the jaws of snails, whelks, and cuttle-fish. Many of the crab class have not one only, but several, pairs of legs following the mouth converted into jaws. Thus, if you examine a big shore-crab, or, better, an edible crab, and a lobster, and a large prawn, you will find that they all have five pairs of legs converted into short foliaceous jaws (hence called "foot-jaws"), and overlying the first very strong pair, or mandibles.

Following these "foot-jaws" you find in a crab or a lobster the great nipping claws and the four large walking legs—the same in proportion and shape in crab, lobster, and prawn, much bigger than the foot-jaws. But the curious thing is that if you set them out and carefully compare them (for they are not simple jointed limbs, but each has two or even three diverging stems carried on a basal joint), you will find a strange and fascinating "likeness in unlikeness," or an agreement of the parts of which they are built, and yet a difference between all of them.

The rings of the body to which the jaw-legs and legs are attached are fused into one unjointed piece. The spine in front of the mouth and the support of the eyes and the feelers or "antennÆ" are fused with that piece. It forms on the back a great shield—often called "the head"—which overhangs and is bent down over the sides of this region, so as to protect the gills, which you can see by cutting away the overhanging flap.

Following on the jaw-legs or foot-jaws and walking-legs, in the three crustaceans we are looking at, comes the jointed tail or hind-body, consisting of seven pieces. The first five rings of the tail have small Y-shaped legs, a pair to each ring. They are called "swimmerets," whilst the sixth has legs of the same shape, but very large and flat. In the middle between these large flat legs is the last ring, which has no legs, but is perforated by the opening of the intestine. You will see if you compare the crab and the lobster (or the prawn, which is very much like the lobster), that the crab has the so-called head (really head and mid-body combined) drawn out from side to side, so as to make it much wider than it is long. And, moreover, the jointed tail or hind-body seems at first sight to be absent in the crab. But if you turn the crab (a dead one) on his back, you will find that he has a complete tail, on the whole like that of the lobster, but pointed and bent forwards, and closely packed under the fused head and mid-body in a groove, from which you can raise it and turn it back.

We have not yet done with the various forms assumed by the legs of our three crustaceans—for, actually in front of the mouth, there are two pairs of peculiarly altered legs. Originally in crab-ancestors, and at the present day in the very minute young stage of growth called "the Nauplius" (Fig. 9), the mouth was not behind these two front pairs. It has sunk back as it were, gradually moved so as to leave the legs in front of it. As we now see them in the crab, lobster, and prawn, the two pairs of legs in front of the mouth are jointed filamentous things—the feelers or antennÆ—very long in prawns and lobsters, short in crabs. In the ancestors of crabs, lobsters, and prawns these feelers were undoubtedly swimming legs. In the "nauplius" stage (Fig. 9) of some prawns, and in many minute crustaceans often called "water-fleas," we find these feelers not acting as mere sensory organs of touch, but relatively strong and large, with powerful muscles, striking the water and making the little creatures bound or jump through it in jerks.

It has been discovered that in the growth from the egg of many crustaceans the young hatches out as a "nauplius" with only three pairs of legs. The front two pairs later gradually grow to be the feelers, the third pair become eventually the mandibles or first pair of jaw-legs. These legs all present themselves at first as active, powerful swimming "oars," beset with peculiar feathery hairs and not in the shape which they later acquire. The kite-shaped nauplius baby-phase, smaller than a small flea, with its three pairs of violently jerking legs, is a very important little beast. It is the existence of this young stage in the growth of barnacles and sea-acorns which has demonstrated that they are crustaceans, that is to say, belong to the crab class. The fixed shell-like barnacles and sea-acorns hatch from their eggs each as a perfect little "nauplius," like that drawn in Fig. 9. They swim about with jerking movements caused by the strokes of the two front legs and of the pair which will become the mandibles. Their limbs have the special form and are beset with the feather-like hairs, and the whole creature has the kite-like shape—characteristic of the nauplius young of other Crustacea. They are indeed indistinguishable from those young. Whilst it was the Army doctor, Vaughan Thompson, who discovered that barnacles are strangely altered "shrimps," it was Darwin who made one of the most interesting discoveries about them—a discovery of which he was always, and rightly, very proud—as I will explain in the next chapter.