Phylum Arthropoda. Subphylum TrilobitaCommon 500 mya (4000 spp) but extinct by 225 myaEach body segment had a biramous appendage (inner for walking)3 body tagmaCompound eyes with ommatidia. . Trilobites flourished for 270 million years.They died.. Subphylum Chelicerata. Include spiders and mites and daddy LL, scorpions, horseshoe crabsLack antennae1st pair of appendages on the prosoma = the chelicerae (adapted for feeding).