  Cells or bacteria carrying cilia on their surface show many striking features
: alignment of cilia in an array, two-phase asymmetric beating for each cilium,
coordination between cilia and existence of metachronal waves with a constant
phase difference between two adjacent cilia. We give simple theoretical
arguments based on hydrodynamic coupling and an internal mechanism of the
cilium derived from the behavior of a collection of molecular motors, to
account qualitatively for these cooperative features. Hydrodynamic interactions
can lead to the alignment of an array of cilia. We study the effect of a
transverse external flow and obtain a two-phase asymmetrical beating, faster
along the flow and slower against the flow, proceeding around an average curved
position. We show that an aligned array of cilia is able to spontaneously break
the left-right symmetry and to create a global average flow. Metachronism
arises as a local minimum of the beating threshold and leads to a rather
constant flow.
