arthropod

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arthropod Member of the largest animal phylum, Arthropoda. Living forms include crustacean, arachnid, centipede, millipede, and insect. The species (numbering well over 1 million) are thought to have evolved from annelids. All have a hard outer skin of chitin that is attached to the muscular system on the inside. The body is divided into segments, modified among different groups, with each segment originally carrying a pair of jointed legs. In some animals, legs have evolved into jaws, sucking organs or weapons. Arthropods have well-developed digestive, circulatory and nervous systems. Land forms use tracheae for respiration.

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