"Everyone," says Dr. Calman, "has some acquaintance with the animals that are grouped by naturalists under the name Crustacea. The edible crabs, lobsters, prawns, and shrimps are at least superficially familiar, either as brought to the table, or as displayed in the fishmonger's.... Many, however, will be surprised that the barnacles coating the rocks on the seashore, the sand hoppers of the beach, and the wood lice of our gardens, are members of the same class. Still less is it suspected that the living species of the group number many thousands, presenting strange diversities of structure and habit, and playing an important part in the general economy of nature."

The great majority of crustaceans are aquatic animals, breathing by gills or by the general surface of the body, having two pairs of "feelers," or antennæ, on the front part of the head, and at least three pairs of jaws. Most crustaceans are hatched from eggs, usually in a form very different from their parents; and they reach the adult state only after passing through a series of transformations quite as remarkable as those that a caterpillar undergoes in becoming a butterfly. All crustaceans, except a few much modified land forms, breathe by means of feathery or platelike gills which are always an appendage of the legs, where they appear as one or more lobes. Colorless blood propelled by the heart wanders into spaces in these lobes, and there lies separated from the water by a mere film of tissue, through which oxygen is absorbed from the water. Most crustaceans are covered, at least in part, by some sort of shelly coat composed of a combination of the horny substance "chitin" with lime, which reaches its highest state in the big lobsters and crabs. This not only protects and gives support to the internal organs, but also to the muscles by which the animal moves. In other words it plays the part of a skeleton. As it does not increase in size after it is once formed, and cannot stretch much, the crab must cast its shell at intervals as it grows. The new covering, which had been formed underneath the old, before molting, is at first quite soft, and the animal rapidly increases in size owing to the absorption of water. The shell then gradually hardens by the deposition of lime salt.

The reader who may not hitherto have understood the difference between "hard" and "soft-shelled" crabs is now instructed; and it is observable that the figurative expression "a hard-shell," when applied to a man, signifies that he must undergo a complete change before his ideas will be enlarged.

The simplest of the crustaceans are those small creatures of the subclass Branchiopoda (gill-footed) that swarm in our waters, both salt and fresh. Lakes, ponds and ditches abound in a variety of minute or even microscopic species that, in gathering food from equally small bits of dead organic matter, as well as from living plants and animalcules, perform an important service as scavengers—a service, in fact, performed by all crustaceans in a greater degree than by any other single group of animals. They also furnish the basis of food for the whole body of aquatic life, since it is upon these minute crustaceans that fish fry, tadpoles, insect larvæ, caddis flies, and so on, must mainly depend. One of them is Daphnia, familiar to keepers of aquariums. Another is Cyclops, a favorite with microscopists and abundant in stagnant ponds, which is a member of the group called copepods that form an important part of the oceanic plankton, where they are the chief consumers of the minute algæ; but they also occur at all depths. In arctic waters the copepods are so abundant that they form the principal part of the food of certain fishes and of the whalebone whales. These, and their minute relatives, the ostracods, produce a large part of the phosphorescence of the sea, and some of them exhibit bright colors.

All these are free swimmers, but nearly related to them are the barnacles (Cirripedia) whose larvæ float about for a time near shore, and then settle down and attach themselves by their hinder parts to a rock or some other support, and begin to secrete an armature of limy overlapping plates that forms a strong cup in which they sit, often in a crowd that whitens a big rock. When the tide is low these sessile "acorn shells" are tightly closed, but when the water returns, bringing its load of invisible food, the animal stands up, as it were, and thrusting out its feathery legs sweeps the water to capture a meal—a beautiful sight to watch. The relation of the plates in the barnacle's cup to those in the coat of the higher Crustacea is more easily seen in the more pelagic "goose barnacle," whose hinder part is extended into a tough, flexible stalk, while the fore part is covered by plates. This kind is fond of attaching itself to floating timber, to ships' bottoms, or even to the surface of whales, and thus floats or is carried all over the watery globe. To it belongs the ridiculous myth of the barnacle geese.

Great numbers of crustaceans of more advanced types live in the open sea, and at all depths; and many of them assume extraordinary shapes. The space between tide marks, and the mud of salt marshes and tidal creeks abound in a wide variety of species, some of which are familiar to everyone who lives at or visits the seashore. Thus the sand and rows of drifted seaweed on all our eastern beaches are likely to harbor flocks of amphipods, well called "sea fleas" or "sand hoppers," which sometimes jump away before you in hundreds as you walk along.

Here, too, are to be found the pretty, burrowing "mole crabs," or "ivory crabs," so called from their shining white jackets; and a host of other species with strange forms and habits haunt the margins of tropical and Oriental seas. All these are bandits, preying on whatever they can catch, and between times guarding themselves from capture by fishes, bigger crabs, and other enemies, by lying in mud burrows, to the bottom of which they are quick to retreat. The big arm of the fiddler crab, held across its face, closes its burrow like a door. One sort, the hermit crab, has all its hinder parts naked, and so backs into an empty snail shell, curling its taillike soft abdomen around the central column of the shell and so dragging it about with it, with its armored head and thorax sticking out of the mouth of the shell. As it grows it becomes too large for its first shell, and from time to time must leave it and find a larger tenement in which to ensconce itself—a perilous transfer. Let me quote some notes I made on a New England shore to give a picture of crustacean life there in summer.

"The lady crabs were plentiful, always alert, and inclined to be pugnacious at our intrusion. The first one I met instantly rose upright at the surface of the water, and when I made an advance it sprang half way out of the water and cracked its pincer claws together as if supposing it would reach, or at any rate frighten me. Perhaps it was my shadow it clutched at so viciously. If so, the crab probably concluded its huge antagonist to be an intangible ghost upon which the most powerful claws could have no effect, for an instant later it backed down —literally and swiftly—to the bottom, and in a twinkling had wriggled tailwise into the mud and out of sight. When with my shovel I routed madame out of that retreat, she indignantly scuttled off too briskly to be followed, and will have great tales to tell of her adventure.

"The stone and fiddler crabs were as common and comical as usual; and I made the acquaintance of a new one called Gebia, which was a small, semi-transparent, bluish white, washed-out, bloodless specimen, shaped somewhat like a crawfish and carrying bunches of roe beneath its abdomen. It looked like a miniature lobster made of glass and filled with milk. Then in the eelgrass there was a funny isopod, called Caprella. It was half an inch or so long, and clung by its hinder feet to the grass, waving its body up and down in search of minute prey. Other isopods and amphipods were exposed by turning over stones or digging in the sand at the edge of the water—small, pale, shapeless crustacea, which are flattened laterally so that they must lie on their sides, and when uncovered will kick about with feet and tail in laughable anxiety to get under something. Under the stones we found the tubes made by a certain species; and when we captured the active little architect and put him in a bucket of clean water, he instantly began to gather grains of sand and stone and to join them together Into a shield under which he might hide. We found that these grains were joined together by spiderlike threads, which the amphipod spins from two pairs of small legs under the middle of his body, secreting a fluid that hardens in the water. Another (Hippa) about the size and shape of a robin's egg, but with a thin shell of mother-of-pearl (so to speak), gave us great amusement by its extraordinary celerity in burrowing, so that we could hardly seize it before it had squirmed down out of reach into the wet sand."

The edible crabs (Cancer) live in the shallow region just below ebb tide, for they cannot endure exposure to air as well as other species, and live by scavenging. The lobsters are inhabitants of still deeper water, especially where it is somewhat rocky, and devour more carrion than living fish. That miniature of the lobster, the fresh-water crawfish, which is also edible, dwells in deep burrows in wet lands—burrows that are really wells half filled with water. Various species of these and other edible forms of Crustacea are found all over the world.