The Swordtails or Moluccan lobsters ( Merostomata , Xiphosuridae , Xiphuridae ) are very eccentric creatures, remnants of an extinct animal world, not closely related to any of the existing groups to be included. In important respects they deviate from the Shellfish, to which most zoologists add them, albeit with some reservations. They draw closer to the Arachnids and more specifically to the Scorpions. Unified with the latter order, they should probably form a separate class.