Another character inherited by all arthropods is a body divided into segments that are often grouped into larger functional units.
Like Cars in a Train
At some point in their lives, all arthropods have bodies that are internally and externally segmented. These segments piggyback on one another, like a series of cars in a train. Each body segment tends to repeat the same suite of structures (for example, a pair of legs, a set of breathing organs, and a set of nerves), often with slight variations down the length of the animal. Usually, sets of segments are grouped into a larger unit, such as the abdomen.
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Which of these animals are segmented?
Segmentation is a complicated topic. Both the onychophoran and mouse show some segmentation as they develop. The onychophoran also has repeated organs that correspond to internal segmentation. However, neither has obvious external segementation as adults.