Invertebrates have muscles similar to the skeletal and smooth muscles of vertebrates, but with some interesting adaptations.
arthropod skeletal muscles
Arthropod skeletal muscles are very similar to vertebrate skeletal muscle.
insect wings
Insect wings actually beat faster than action potentials arrive from the CNS since the flight muscles are capable of independent, rhythmic contractions.
Clams
The thick filaments of muscles in clams which hold the shell closed contain paramyosin; this unique protein allows the muscles to stay in a fixed state of contraction for up to a month.