Segmentation
Body is “assembled” from a succession of similar segments
Segmentation starts from mesoderm but may later involve endo- and ectodermal tissues as well
Each segment may develop complete sets of adult organs (annelids); damage of one segment is less fatal since other segments duplicate its function
Locomotion is easier when segments can move independently
In adults, segments may be merged (insects, crustaceans); segments are visible in embryological development, though
Vertebrates: segmentation of backbone and muscles
True segmentation: annelids, arthropods, chordates