Arthropoda
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Arthropod, animal with a hard, outer skeleton and a jointed body and limbs. Arthropods make up a phylum of invertebrates that includes insects, such as ants, beetles, and butterflies; crustaceans, such as lobsters, shrimps, and crabs; and arachnids, including scorpions, spiders, and ticks. In terms of sheer numbers and the variety of niches they fill, arthropods are the most successful animals on Earth. More than one million arthropod species have been identified—more than 20 times the number of known fish, amphibian, reptile, bird, and mammal species combined. This figure is considered a low estimate of the phylum's actual size because many arthropod species have yet to be discovered and documented. Some scientists suggest the number of arthropod species in tropical forests alone may approach six million to nine million.
Traditionally, living arthropods are classified among three subphyla: the Chelicerata, Crustacea, and Unirama. Trilobites, a fourth subphylum, flourished during the Cambrian and Ordovician periods of the Paleozoic Era (570 million to 435 million years ago) but became extinct by the mid-Permian Period (about 250 million years ago), for reasons that are not fully understood.
Chelicerata, named for their chelicerae, or specialized mouthparts, include scorpions, spiders, ticks, mites, and horseshoe crabs. These creatures have six pairs of appendages, one or two body regions, and lack wings and antennae. Most live on land, although a few are aquatic.
With a few exceptions, the Crustacea are aquatic, occupying both freshwater and saltwater niches. Familiar crustaceans include shrimps, lobsters, and crabs. All crustaceans have ten limbs, three body regions, and two pairs of antennae. Many of the smallest crustaceans are zooplankton—drifting or weak-swimming animals that form the base of most marine food webs.
The Unirama include centipedes, millipedes, and insects. Centipedes have wingless, multisegmented bodies, with one pair of legs on each body segment. The bodies of millipedes are also wingless and multisegmented but have two pairs of legs per segment. Insects have three pairs of legs, one pair of antennae, and may be wingless or have one or two pairs of wings. The majority of insects are land-dwellers, although some live in freshwater or marine environments.
Scientific classification: Arthropods make up the phylum Arthropoda, which is divided into three living subphyla: Chelicerata, Crustacea, and Unirama.
Arthropod, animal with a hard, outer skeleton and a jointed body and limbs. Arthropods make up a phylum of invertebrates that includes insects, such as ants, beetles, and butterflies; crustaceans, such as lobsters, shrimps, and crabs; and arachnids, including scorpions, spiders, and ticks. In terms of sheer numbers and the variety of niches they fill, arthropods are the most successful animals on Earth. More than one million arthropod species have been identified—more than 20 times the number of known fish, amphibian, reptile, bird, and mammal species combined. This figure is considered a low estimate of the phylum's actual size because many arthropod species have yet to be discovered and documented. Some scientists suggest the number of arthropod species in tropical forests alone may approach six million to nine million.
Traditionally, living arthropods are classified among three subphyla: the Chelicerata, Crustacea, and Unirama. Trilobites, a fourth subphylum, flourished during the Cambrian and Ordovician periods of the Paleozoic Era (570 million to 435 million years ago) but became extinct by the mid-Permian Period (about 250 million years ago), for reasons that are not fully understood.
Chelicerata, named for their chelicerae, or specialized mouthparts, include scorpions, spiders, ticks, mites, and horseshoe crabs. These creatures have six pairs of appendages, one or two body regions, and lack wings and antennae. Most live on land, although a few are aquatic.
With a few exceptions, the Crustacea are aquatic, occupying both freshwater and saltwater niches. Familiar crustaceans include shrimps, lobsters, and crabs. All crustaceans have ten limbs, three body regions, and two pairs of antennae. Many of the smallest crustaceans are zooplankton—drifting or weak-swimming animals that form the base of most marine food webs.
The Unirama include centipedes, millipedes, and insects. Centipedes have wingless, multisegmented bodies, with one pair of legs on each body segment. The bodies of millipedes are also wingless and multisegmented but have two pairs of legs per segment. Insects have three pairs of legs, one pair of antennae, and may be wingless or have one or two pairs of wings. The majority of insects are land-dwellers, although some live in freshwater or marine environments.
Scientific classification: Arthropods make up the phylum Arthropoda, which is divided into three living subphyla: Chelicerata, Crustacea, and Unirama.
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