Massachusetts General Laws Part III. Courts, Judicial Officers and Proceedings in Civil Cases (Ch. 211-262) Ch. 223, § 19

In a civil action if the name of the defendant is not known to the plaintiff, the defendant may be designated in the summons and complaint by a fictitious name. The use of a fictitious name shall not be grounds for dismissal of the complaint if sufficient service has been made, and amendments shall be allowed and terms may be imposed.