Those are the planaria (aka planarian) that you dissected in ninth grade biology, and they are not good for the garden. They're introduced, and if you find one, don't squash or cut it because it just becomes that many new ones. Seal it in plastic and throw it in the trash, or spray it with orange oil and let it shrivel and melt away.

Land Planarian from the dirtdoctor.com site.

Planaria reproduction.

SRS