Class Merostomata (Horseshoe Crabs)
a) All Eurypterids are now extinct. They are considered ancestors of horseshoe crab and scorpion, which accounts for their common name giant water scorpion.
Subclass Xiphosurida - horseshoe crabs
Only five species survive today. Found in all oceans, but most abundant in polar waters.
a) Unsegmented, horseshoe-shaped carapace (hard dorsal shield)
b) broad abdomen w/ long telson (tailpiece)
1) six pairs broad, thin appendages fused in median line
c) Cephalothorax bears five pairs of walking legs and pair chelicerae
d) Book gills (flat leaflike gills) are exposed on some abdomen appendages
e) Two compound and two simple eyes on carapace.
Swim by means of abdominal plates and can walk with its walking legs. Feeds on worms and small molluscs at night. It seizes w/ chelicerae.