Diversity
Many produce exoskeletons, which are composed of a range of materials. Bone, cartilage, or dentine is used in the Ostracoderm fish and turtles. Chitin forms the exoskeleton in arthropods including insects, arachnids such as spiders, crustaceans such as crabs and lobsters (see arthropod exoskeleton), and in some fungi and bacteria. Calcium carbonates constitute the shells of molluscs (see Mollusc shell), brachiopods, and some tube-building polychaete worms. Silica forms the exoskeleton in the microscopic diatoms and radiolaria.
Some organisms, such as some foraminifera, agglutinate exoskeletons by sticking grains of sand and shell to their exterior. Contrary to a common misconception, echinoderms do not possess an exoskeleton, as their test is always contained within a layer of living tissue.
Exoskeletons have evolved independently many times; 18 lineages evolved calcified exoskeletons alone. Further, other lineages have produced tough outer coatings analogous to an exoskeleton, such as some mammals – (constructed from bone in the armadillo, and hair in the pangolin) – and reptiles (turtle and Ankylosaur armor are constructed of bone; crocodiles have bony scutes and horny scales).
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Famous quotes containing the word diversity:
“We call the intention good which is right in itself, but the action is good, not because it contains within it some good, but because it issues from a good intention. The same act may be done by the same man at different times. According to the diversity of his intention, however, this act may be at one time good, at another bad.”
—Peter Abelard (1079–1142)
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—James Madison (1751–1836)