Linked from
The 15 pages that link to Horseshoe crab, each with the reason it gives.
ArachnidCompared with: It is a close chelicerate relative, but it is not classified as an arachnid.
CheliceraBroader topic: Its chelicerae handle food rather than forming prominent grasping claws.
Benthic locomotionBroader topic: Its walking legs move it across tidal flats and shallow seabeds.
ProsomaBroader topic: Its large prosoma carries the eyes and most appendages.
TelsonBroader topic: Its conspicuous telson helps the animal right itself when overturned.