Living descendents of the Trilobites
Although they radiated throughout the oceans, only one descendent of this group survives today, the horse-shoe crab (Limulus). This animal is larger than its ancestral trilobites, and segmentation of its armour have fused to form a large domed shield. These animals generally live at great depths but each spring they migrate towards the coast and during full moon and high tides they drag themselves onto the beach where they copulate. Today the similarities between the horse-shoe crabs and the trilobites are only evident in the larval stage where segmentation of the armour plates are clearly discernable in the horse-shoe crab larvae.
Underside of a female showing the legs and book gills. When encountering a horseshoe crab in this position, one should "just flip 'them" and turn the crabs back over.