  Muscular hydrostats, such as octopus arms or elephant trunks, lack bones
entirely, endowing them with exceptional dexterity and reconfigurability. Key
to their unmatched ability to control nearly infinite degrees of freedom is the
architecture into which muscle fibers are weaved. Their arrangement is,
effectively, the instantiation of a sophisticated mechanical program that
mediates, and likely facilitates, the control and realization of complex,
dynamic morphological reconfigurations. Here, by combining medical imaging,
biomechanical data, live behavioral experiments and numerical simulations, we
synthesize a model octopus arm entailing ~200 continuous muscles groups, and
begin to unravel its complexity. We show how 3D arm motions can be understood
in terms of storage, transport, and conversion of topological quantities,
effected by simple muscle activation templates. These, in turn, can be composed
into higher-level control strategies that, compounded by the arm's compliance,
are demonstrated in a range of object manipulation tasks rendered additionally
challenging by the need to appropriately align suckers, to sense and grasp.
Overall, our work exposes broad design and algorithmic principles pertinent to
muscular hydrostats, robotics, and dynamics, while significantly advancing our
ability to model muscular structures from medical imaging, with potential
implications for human health and care.
