  We investigate the dynamics of \textbf{\textit{Lumbriculus variegatus}} in
water-saturated sediment beds to understand limbless locomotion in the benthic
zone found at the bottom of lakes and oceans. These slender aquatic worms are
observed to perform elongation-contraction and transverse undulatory strokes in
both water-saturated sediments and water. Greater drag anisotropy in the
sediment medium is observed to boost the burrowing speed of the worm compared
to swimming in water with the same stroke using drag-assisted propulsion. We
capture the observed speeds by combining the calculated forms based on
resistive-force theory of undulatory motion in viscous fluids and a dynamic
anchor model of peristaltic motion in the sediments. Peristalsis is found to be
effective for burrowing in non-cohesive sediments which fill in rapidly behind
the moving body inside the sediment bed. Whereas, the undulatory stroke is
found to be effective in water and in shallow sediment layers where anchoring
is not possible to achieve peristaltic motion. We show that such dual strokes
occur as well in the earthworm \textbf{\textit{Eisenia fetida}} which inhabit
moist sediments that are prone to flooding. Our analysis in terms of the
rheology of the medium shows that the dual strokes are exploited by organisms
to negotiate sediment beds that may be packed heterogeneously, and can be used
by active intruders to move effectively from a fluid through the loose bed
surface layer which fluidize easily to the well-consolidated bed below.
