Crustaceans are so diverse that their single defining characteristic is having two pairs of antennae. Also, most crustaceans have a pair of jawlike, chewing mouthparts called mandibles. In most crustaceans, each body segment has a pair of appendages, and at least some of those appendages are branched. Although some crustaceans have 60 or more body segments, most crustaceans have 16 to 20 segments, which are fused into several tagmata.
Some small crustaceans, such as pill bugs, respire through the thin areas of their exoskeleton. Larger crustaceans, such as crayfish and lobsters, use gills to respire. The exoskeletons of aquatic crustaceans, such as lobsters, often contain large amounts of calcium carbonate and thus are extremely hard.
During the development of many crustaceans, the embryo becomes a free-swimming larva called a nauplius (NAH-plee-uhs), which looks quite different from an adult of the species. As Figure 36-4 shows, a nauplius has three pairs of appendages and a single eye in the middle of its head. Through a series of molts, the nau-plius eventually takes on the adult form.
Appendages figure 36-4
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