Crustacea, soft-shelled aquatic animals, as the lobsters, crabs, shrimps, etc. Aristotle gave the name to this group, to distinguish it from that of the harder-shelled animals, which he called or testacea, the mollusca of our present system of arrangement. In this system the Crustacea constitute one class of the primary division articulata of the animal kingdom, and the term malacostraca has been retained for one great section of the class, while another is called the entomostraca, or shelled insects. The subdivisions of these sections are variously presented by different authorities. Those of the malacostraca, as given by Milne-Edwards, are generally adopted. They are as follows:
1. Eyes on peduncles, and movable; feet 10.
Brachyura, short-tailed - as crabs.
Anomura - as hermit crabs, etc.
Umpeltata - as squilla.
Bipeltata - as phyllosoma, lucifer, etc.
2. Eyes sessile and immovable; feet 14.
Amphipoda - as gammarus, etc, having feet simple and claw-shaped. Laemodipoda - as leptomera, etc. Jsopoda - as oniscus or wood louse, armadillo, etc.
The entromostraca are divided by the same authority into the Orders.
Phyllopoda - as apus, etc. Cladocera - as daphnia, etc.
4. Entomostraca Proper.
Copepoda - as cyclops, etc. Ostrapoda - as cypris, etc.
Arancei/brmes - extremities long and slender, adapted for walking. Siphonostomata - extremities not adapted for walking. Lernaeiformes - extremities rudimentary.
The Crustacea are furnished with organs of respiration fitted, unlike those of the other ar-ticulata, for use beneath the surface of the water; and they are provided with a shell which is either a horny tegument, as in the case of the shrimp, or a calcareous crust, as in the lobster - not stony like those of the mollusca. It is in fact an external articulated skeleton secreted from their own bodies, and periodically thrown off and renewed. In casting its shield the animal is said to pine away and become smaller, until at last it readily slips out of its covering. Sometimes when caught they voluntarily cast off a limb by which they are held, and which they often can very well spare, having always at least four pairs besides a pair of claws. In some genera they are so numerous that the animals approximate the my-riapoda. A lost limb too is replaced, gaining in growth at each moulting, while the body is unconfined. These organs, with those of respiration, and the tail also, are supported by the body; the antennae or feelers, eyes, and mouth belong to the head.
The organs of sight and touch are remarkably well developed; and it is not a little interesting to find in those very ancient representatives of this class, the trilo-bites, whose period of existence was as remote as that of the formation of the older Silurian rocks, the same peculiarities and perfection in the structure of the eye as are seen in the highly complicated organization of that of the fly and the butterfly of the present day. The organs of hearing, it is thought, may be detected in some genera of the decapoda, and the habits of many of the Crustacea seem to imply the possession of the sense of smell. The shelly covering corresponds in its protuberances and depressions to the form of the important organs of the body within. The progressive motion of the animals is sometimes by walking, side-wise, backward, or forward; by climbing, as seen in their progress over the weeds and rocks at the bottom of the water; by swimming, and also by leaping. The lobster, clumsy as he appears, and loaded with his heavy claws, is often seen to dart backward by suddenly flapping his tail toward the thorax, throwing himself a distance of more than 20 feet with the swiftness of a bird or a dolphin.
From the perfection of his sight he can dart like a mouse directly into his hole, scarcely large enough to admit his body. The young shrimps on the ebb of the tide are often seen along the shallow margin of the water, as observed by Paley in his "Natural Theology," skipping into the air in such numbers that they resemble a cloud or thick mist hanging over the edge of the water to the height of half a yard. The trilobites were fitted by their organization for swimming just beneath the surface of the water, and with the back downward. Like the isopod crustacean, the wood louse, they possessed the faculty of rolling themselves into a ball as a defence against attack from above. The Crustacea are found mostly in salt water; some species, however, live in lakes and rivers, and a few upon the land. Some of them are of considerable size, the largest being the lobsters; but they are generally very small. The salt water is almost filled with varieties of them so minute that they are rarely observed, and it is said that a portion taken up at random will always be found to contain a number of them.
Numerous species furnish food for man, and all are preyed upon by the inhabitants of the deep.
Some species of the whale subsist upon minute Crustacea drawn in swarms into their huge mouths, and caught in the fibrous web that lines them, while the water is ejected. Many of the terrestrial Crustacea, as the land crabs, are said to visit the sea periodically to deposit their spawn. They burrow also in the mud and in damp places, and their gills are always moist. The oniscus, or wood louse, has no such arrangement of the gills, and is consequently confined to damp places. Some species of the anomura or hermit crabs, known also as soldier crabs, are found living in the sea, and others upon the land. The entomostraca are mostly fresh-water, many of them microscopic. They subsist upon animalcules and microscopic plants. In their progress from the egg to maturity some of them, as the cyclops, undergo curious transformations. Some live in salt water, and one species, the hranchipus stagnalis, called also the brine worm, lives in the concentrated solutions of salt, such as those of the brine pans of salt works, which contain two pounds of salt to a gallon of water. Some, like the fresh-water cyclops, sustain intense cold without injury, being sometimes frozen into the ice, and coming out when it melts as active as ever.
Many are parasites, as the lerneas, and are classed by themselves by some naturalists, with the name epizoa. The lerneonemia moni-laris infests the head of the sprat, attaching itself near the eye. It is luminous in the dark, and the fishermen say that a shoal of sprats is often headed by those thus infested, which they call lantern sprats. The cod also, and other large fish of our deep waters, have their parasitical Crustacea. The limuli, or king crabs, or "horse-shoes," common upon our coast, are placed by Milne-Edwards in a sub-class, which he calls xiphosura. It differs from the other genera by having no organs for conveying food to the mouth. The name is from a sword, with reference to the long, pointed, spear-like appendage usually called the tail. This is used by the natives of the Moluccas to point their weapons. A buckler entirely hides the limbs and organs of the animal as he moves along upon the sand, or in the water upon the bottom. They are found as fossils in the coal and Jura formations of Europe. - The Crustacea furnish a great number of species much esteemed as food by man, as the lobster, crabs, shrimp, prawn, etc.; and the business of capturing them is extensively pursued.