Endostyle in the context of "Lamprey"

A chordate (/ˈkɔːrdeɪt/ KOR-dayt) is a bilaterian animal belonging to the phylum Chordata (/kɔːrˈdeɪtə/ kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail.

In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates. These CSIs provide molecular means to reliably distinguish chordates from all other animals.

The dorsal nerve cord is an anatomical feature found in all chordates, mainly in the subphyla Vertebrata and Cephalochordata, as well as in some hemichordates. It is one of the five embryonic features unique to all chordates, the other four being a notochord, a post-anal tail, an endostyle, and pharyngeal slits.

All chordates (vertebrates, tunicates and cephalochordates) have dorsal hollow nerve cords.

Tunicates are marine invertebrates belonging to the subphylum Tunicata (/ˌtjuːnɪˈkeɪtə/ TEW-nih-KAY-tə). This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals.

Despite their simple appearance and very different adult form, their close relationship to the vertebrates is certain. Both groups are chordates, as evidenced by the fact that during their mobile larval stage, tunicates possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, post-anal tail, and an endostyle. They resemble a tadpole.