Lobsters are joint-legged animals that belong to the phylum Arthropoda. This diverse phylum of animals also includes the insects, spiders, ticks, mites, millipedes and centipedes. Phylum characteristics include a chitinous exoskeleton, jointed appendages, a ventral nervous system, and a dorsal brain. Lobsters are crustaceans, which are placed in the class Malacostraca along with the shrimp. Crabs differ from lobsters in having a broad upper shell or carapace and an abdomen that is shorter and either tapered or tucked forward beneath the carapace. Because they have five pairs of legs, crabs and lobsters belong to the order Decapoda. The first one or two pairs of legs are typically enlarged and modified as chelicera, which are variously used for feeding or defense. In some decapods they may also have a sexual function. The remaining three or four pairs of legs are used for walking, or, in some species, swimming or climbing.
Millions of American lobsters (Homarus americanus) are harvested for food each year, making them possibly the most commercially important of all crustaceans. Lobsters differ from crabs in having an extended thorax. Most are large: up to sixty centimeters in length and twenty-two kilograms in weight. The first pair of walking legs is modified into large claws or chelipeds. The European lobster (Homarus gammarus) is similar in shape but somewhat smaller. Lobsters walk forward using their walking legs, but when threatened swim rapidly backward using their broad tail segment or telson. Ecologically, lobsters are creatures of rocky subtidal zones that rarely extend activities shoreward. Eggs are laid in late summer, generally July and August of every other year, and are glued to the female's abdominal appendages. They hatch in about a year and the young remain attached for some time before drifting free and becoming part of the plankton. The spiny and slippery lobsters of the tropics and pantropics are not closely related to the American lobster. Some reach the size of American lobsters, but lack pincers. Spiny lobsters are noted for their mass migrations during fall, in which up to fifty individuals march together, heads-to-tails, in long queues. Females are capable of producing four million eggs during the spring reproductive season. The transparent juveniles attach to jellyfish and undergo a series of molts for about a year, gradually transforming into a five-centimeter lobster. Lobsters actively forage for live prey, including worms, mollusks, crabs, and small fishes, as well as organic debris.
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