A retractor in accordance with embodiments of the invention includes an adapter and an arm that can be releasably connected to the adapter. An end portion of the arm includes first and second legs and a lock well between the legs. The adapter includes a tubular channel having side walls configured to receive the end portion of the arm, and a lock having a cam in the channel that is rotatable between a release position and a lock position. The lock is configured such that (1) when the lock is in the release position the end portion of the arm can be inserted into the channel and removed from the channel with the cam passing to and from the lock well between the legs, and (2) when the lock is in the lock position the cam is in the lock well and engages the legs to urge the legs into engagement with the side walls of the channel.
