The Animal Axis1

Synopsis. Soft-bodied animals are an answer to the problem, solved for each species over evolutionary time, to design a cylindrical, motile machine composed of pliant polymeric materials (collagen and glycoproteins) and actuated by a contractile polymer (actomyosin). The vertebrate body is a cylindrical set of pliant collagenous membranes. Axial notochords and backbones occur where membranes intersect. The basis for all vertebrate architecture is the collagen fiber that best functions to resist tension. Axial compressive forces in notochords and backbones occur as tensile stresses in collagen fibers in intervertebral discs and zygapohyseal ligaments. Bone provides local stiffening where muscles pull. Large muscle masses apply large forces via tendons thus allowing for leverage in the function of axes of bodies and appendages. Although isolated species in invertebrate phyla have notochord- or backbone-like structures, only echinoderms and vertebrates have a central axis to resist axial compression. Design is a useful tool in forming scientific hypotheses.

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