  The static potential is investigated in string and membrane theories coupled
to $U(1)$ gauge fields (specifically, external magnetic fields) and with
antisymmetric tensor fields. The explicit dependence of the potential on the
shape of the extended objects is obtained, including a careful calculation of
the quantum effects. Noting the features which are common to the dynamics of
strings and membranes moving in background fields and to the swimming of
micro-organisms in a fluid, the latter problem is studied. The Casimir energy
of a micro-organism is estimated, taking into account the quantum effects and
the backreaction from the outside fluid. The static potential is investigated
in string and membrane theories coupled to $U(1)$ gauge fields (specifically,
external magnetic fields) and with antisymmetric tensor fields. The explicit
dependence of the potential on the shape of the extended objects is obtained,
including a careful calculation of the quantum effects. Noting the features
which are common to the dynamics of strings and membranes moving in background
fields and to the swimming of micro-organisms in a fluid, the latter problem is
studied. The Casimir energy of a micro-organism is estimated, taking into
account the quantum effects and the backreaction from the outside fluid.
