The word "Arthropod" [ar'thrO pod] comes from the Greek arthron, or joint, plus podos, or foot.

Arthropods make up about 80 per cent of all known animals. They include the largest of all animal classes, the insects, as well as the various crustaceans, centipedes and millipedes, plus the arachnids, sea spiders and king crabs.

Characteristics (1) Chitinous skeleton; (2) jointed legs; (3) segmented body.

Size Microscopic to over five feet

Habitat Land, sea and air.

Other Characteristics Typically, the external skeleton of chitin is divided into connected plates and cylinders so that the animals can move with maximum freedom. The periodic shedding and secretion of a new skeleton, which is at first soft and pliable and can be stretched, enables the arthropods to increase in size.

Class Arachnida -- spiders, scorpions, daddy longlegs, ticks, mites -- Almost all arachnids are terrestrial, have four pairs of walking legs, simple eyes and lack gills. The head has no antennae and, instead of mandibles, has a pair of appendages for handling prey.

Class Pycnogonida -- sea spiders -- These marine animals get their common name from their spiderlike appearance. Though most coastal species do not grow larger than an inch and a half, there are sea spiders inhabiting the deep sea that have leg spans of two feet.

Class Merostomata -- horseshoe crabs -- These large-shelled scavengers are found on thebeaches of the northwest Atlantic coast and the Gulf of Mexico, as well as along most Asian shores.

Class Crustacea -- shrimps, crabs, barnacles, lobsters -- over 30,000 species -- Crustaceans possess two pairsof antennae, mandibles and other head appendanges called maxillae which aid in feeding. The trunk usually bears pairs of appendages used for locomotion. They have compound eyes. Generally an abdomen and thorax, which may be covered by a carapace, are present, but the number of body segments is not the same for all groups. Plentiful in most seas, some crustaceans also inhabit fresh water and land.

Class Diplopoda -- millipedes -- over 8,000 species -- Millipedes have two pairs of legs on almost every segment. Slow-moving creatures, living under stones, bark, piles of leaves and logs, some species curl up when threatened, while others may defend themselves by secreting a repelling fluid.

Class Chilopoda -- centipedes -- 3,000 to 5,000 species -- Centipedes have a pair of legs on almost every segment of the body. Carnivorous and agile, they carry poison-bearing claws beneath the head, with which they kill small prey.

Class Insecta -- dragonflies, beetles, grasshoppers, bees -- Largest of all animal classes, the insects comprise about three quarters of all described species of animals living today. Though vastly different in their many adaptations, all insects possess a single pair of antennae, a body that is divided into three major parts (head, thorax and abdomen) and three pairs of segmented legs. The skeleton forms the surface of the body, and the circulatory system does not rely upon blood to transport oxygen.

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Science.--Zoology.--Invertebrates.--Phylum Arthropoda.

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