Segmentation

As the lab manual explains, segmentation is the characteristic that the body is divided into repetitive segments arranged on a longitudinal axis. Each segment contains parts of the circulatory, digestive, nervous, and excretory systems. Annelids are among the earliest organisms to appear with a segmented body plan.

Other organisms also have a segmented body plan. Arthropods, such as insects, show a strong degree of segmentation with a head thorax and abdomen with specific functions. Echinoderms, such as starfish, are also segmented, but their segmentation is not along a longitudinal axis because of their radial symmetry.

Chordates, such as humans, are also segmented, but our segments are so specialized that we barely notice it. On humans, the upper jaw, lower jaw, the hyoid bone (found in the throat), vertebrae, and ribs are all remnants of our segmented body form.