Massachusetts General Laws Part III. Courts, Judicial Officers and Proceedings in Civil Cases (Ch. 211-262) Ch. 246, § 33

If a person claiming, by assignment from the defendant or otherwise, goods, effects or credits in the hands of a supposed trustee enters an appearance, he shall be admitted as a party in order to determine his title to such goods, effects or credits, and may allege and prove any facts which have not been stated or denied by the supposed trustee. Such allegations shall be tried and determined as provided in section seventeen upon depositions or oral testimony as the court orders. If he does not voluntarily enter an appearance, the court may issue an order of notice to him.