Simplify the following text for a Grade 8 native English-speaking person.
Original: Xiphosura is an order of marine chelicerates which includes a large number of extinct lineages and only four recent species in the family Limulidae , which include the horseshoe crabs .
Simplified: Xiphosura is an order of marine chelicerates . It includes a large number of extinct lines and only four recent species in the family Limulidae , which include the horseshoe crabs .