The horseshoe crab, scientific name Limulus polyphemus (Linnaeus, 1758), is part of the chelicerata subphylum not the crustacea. There is a species of this amongst the crustacean the collection here at the Manchester Museum, which is what lead me to research more into this subphylum.
Differences which set chelicerata apart from crustaceans include chelicerata having uniramous limbs where as crustaceans have biramous limbs. A uniramous limb comprises a single series of segments attached end-to-end. A biramous limb, however, branches into two, and each branch consists of a series of segments attached end-to-end.
Crustaceans have 6 pairs of legs whereas chelicerata have 6 pairs.
I have photographed our limulus polyphemus specimen below for anyone interested.