Chordate

Chordata is a phylum of animals that includes the vertebrates and several closely related invertebrate groups. Members share a set of embryonic features, although some are only present during development or in particular life stages. The defining characteristics are the notochord, a flexible, longitudinal rod; a dorsal hollow nerve cord; pharyngeal slits or pouches; an endostyle or thyroid gland; and a post-anal tail. In many chordates, these features are evident during embryonic development and may be modified or lost in adulthood.

Chordates are divided into three subphyla: Cephalochordata (lancelets), Urochordata (tunicates), and Vertebrata (also called Craniata). Cephalochordates retain all key features throughout life; tunicates in adulthood lack many chordate traits but their larvae exhibit the full set; vertebrates possess a cranium and a vertebral column, and include fishes, amphibians, reptiles, birds, and mammals.

Chordates are deuterostomes, with development characterized by radial cleavage and the anus forming from the second opening. The endostyle of non-vertebrate chordates is homologous to the thyroid gland of vertebrates. The group has a wide range of forms and lifestyles, from sessile filter-feeders to highly active vertebrates with complex organ systems. The fossil record includes early Cambrian examples such as Pikaia and related forms.