When the name of the defendant is unknown to the plaintiff, the writ may be issued against him by a fictitious name, and, being duly served, it shall not for that cause be abated, but may be amended on such terms as the court may order.
RSA 509:7
RS 182:9. CS 193:9. GS 203:7. GL 222:7. PS 218:7. PL 330 :7. RL 386:7.