Protochordates have two subphyla: urochordates and cephalochordates. Urochordates include sea squirts and related invertebrate groups, while cephalochordates are represented by amphioxus. Amphioxus is considered the most basal chordate and shares several anatomical similarities with the larval stage of lampreys, including a dorsal nerve cord, brain, myomeres, and filter feeding behavior, though amphioxus lacks a heart and organized kidney.