^Thorax^: The middle part of the body. [pg244] ARTHROPODA This subdivision of the animal kingdom includes insects, centipedes, spiders, and crustaceans, which together constitute more than half the known species of animals. Although these animals are so unlike in general appearance, it is easy to recognize the common characteristics which place them together in one group. The name /Arthropoda/, meaning "jointed-footed," suggests perhaps one of the most obvious points of resemblance. The /Arthropoda/ have bilateral symmetry, one side of the body being like the other; they are covered with a horn-like material (chitin); they are divided into segments; the segments have appendages; and the appendages are jointed so as to admit freedom of motion. Their manner of growth is peculiar; they cast off their rigid external coverings and secrete larger coats of mail, and at these periods increase in size or undergo metamorphosis. There are such modifications of these general features as the habits of the different species demand. For instance, the appendages may be constructed for walking, swimming, boring, sucking, or the seizure and preparation of food. In some animals the appendages form a part of the breathing-organs, in others are used as organs of sense. Every detail of the organism, down to the hairs, has its special use and function. CLASS =CRUSTACEA= The crustaceans vary in size from microscopic minuteness to two feet or more in length. The giant crab of Japan (/Kaempferia kaempferi/) exceeds this, being commonly from eight to twelve feet [pg245] across the arms, and is said to reach even greater proportions. Crustaceans live on land and sea, and in both fresh and salt water; they may be parasitic, sedentary, or free and active. There are said to be over ten thousand species, which include crabs, lobsters, shrimps, beach-fleas, wood-lice, barnacles, and water-fleas. Nine tenths of the species are marine; of these some are pelagic, and their transparent forms constitute a part of the plankton. Others live on the bottom in deep water and attain a large size. A vast number live in the littoral zone and form a considerable part of that crowded community. They are scavengers and freebooters, being great fighters, as well as predatory in their habits. As a rule, they feed on dead organic matter, which they consume in surprising quantities; some have also a vegetarian diet. They have, like starfishes, the singular power of throwing off their limbs and growing new ones. The /Crustacea/ derive their name from the nature of their crust or covering. This is earthy and brittle, not stony, like the shells of mollusks. This crust, or exoskeleton, has various degrees of firmness, from that of a delicate polished cuticle, seen in small forms, to the heavy armor of some crabs and the shell-like character of the covering of barnacles.