Earthworms have two muscles which is used in locomotion. Both muscles are circular and longitudinal (inner layer of the series of muscles). The diameter of the earthworms body is reduced when the circular muscles are contracted which makes them appear to be thin. The worm appears short when the longitudinal muscles contract thus, its length is reduced.
Amphioxus on the other hand, have two unique muscle fiber types. This includes deep fibers and smaller but mitochondria-rich superficial fibers. Each of which has its own innervation.