Axial systems and their actuation: new twists on the ancient body of craniates

Craniate animals--vertebrates and their jawless sister taxa--have evolved a body axis with powerful muscles, a distributed nervous system to control those muscles, and an endoskeleton that starts at the head and ends at the caudal fin. The body axis undulates, bends, twists, or holds firm, depending on the behavior. In this introduction to the special issue on axial systems and their actuation, we provide an overview of the latest research on how the body axis functions, develops, and evolves. Based on this research, we hypothesize that the body axis of craniates has three primary, post-cranial modules: precaudal, caudal, and tail. The term "module" means a portion of the body axis that functions, develops, and evolves in relative independence from other modules; "relative independence" means that structures and processes within a module are more tightly correlated in function, development, and behavior than the same processes are among modules.