C. elegans is capable of coordinated locomotion both when swimming in water and when crawling on an agar surface, two behaviors with distinct kinematics. By analyzing the worm''s locomotion in a range of fluids with increasing visco-elasticity, we were able to demonstrate that swimming and crawling are merely two snapshots out of a continuum of locomotory behaviors that are achieved by a modulation of a single gait[1]. This finding suggests that a single neural mechanism underlies this entire range of behaviors. We developed an integrated model of the worm''s forward locomotion that consists of a ventral cord nervous system, muscles, a body and an environment. The neural model consists of excitatory B-like bistable neurons that receive sensory feedback (mediated, in the model, by posteriorly directed stretch receptors on B class motorneurons) as well as inhibition from D-type neurons. Muscles receive both excitatory and contralateral inhibitory inputs and control the shape of a worm, instantiated by a physical model of the C. elegans body, embedded in a model of the visco-elastic environment. The integrated model can generate and coordinate oscillations, and captures the entire swim-crawl transition as the properties of the environment are changed. The neuromuscular control of the locomotion is modulated solely by the stretch receptor input which in turn varies with the external physics. No parameters are changed in the neuromuscular control. The model does not require a central pattern generator (or distinct neural mechanism) in the head, and can start from arbitrary initial body shape. Tests of the model are presented in virtual knockout and laser ablation simulation experiments, as well as in complex environments such as microfluidic "artificial dirt" chips and irregular granular media, which the worm is likely to encounter in its natural habitat. We are now using the model to gain insight into various uncoordinated phenotypes, including those that have a stronger effect in liquid than on agar. [1] Berri S, Boyle JH, Tassieri M, Hope IA and Cohen N. 2009. "Forward locomotion of the nematode C. elegans is achieved through modulation of a single gait." HFSP journal, In press.