The answer to whether all chordates are vertebrates is no. Vertebrates (fish, birds, and mammals) are a well-known subphylum within the much larger Phylum Chordata. Chordata encompasses any animal that possesses a specific set of four anatomical features at some point during its life cycle. While all vertebrates are chordates, the reverse is not true, as the phylum also contains two groups of marine invertebrates. These invertebrates share the foundational chordate body plan but lack the bony or cartilaginous support structure that defines vertebrates.
The Four Traits of Chordates
The Phylum Chordata is defined by the presence of four distinct physical characteristics, which may only appear during the embryonic or larval stage in some species. The first is the notochord, a flexible, rod-shaped structure composed of fluid-filled cells encased in a firm sheath. This structure provides skeletal support along the length of the body, offering a rigid but flexible surface for muscle attachment.
Positioned directly above the notochord is the dorsal hollow nerve cord, which develops from a plate of ectoderm that rolls into a tube. This tube ultimately forms the central nervous system, including the brain and spinal cord. The third feature is the presence of pharyngeal slits, which are openings in the pharynx behind the mouth. In aquatic species, these slits function in filter-feeding or respiration. In terrestrial vertebrates, they are present only during embryonic development and give rise to structures of the ear and throat.
Finally, all chordates possess a post-anal tail, a muscular extension of the body that continues past the anus. This tail contains skeletal elements and muscle tissue, providing locomotion in many aquatic species. In humans and some other apes, this structure is reduced to a vestigial tailbone (coccyx), but it is present during the embryonic stage.
The Defining Feature of Vertebrates
The Subphylum Vertebrata is distinguished by a specialized modification of the chordate body plan. The most recognizable distinction is the development of a segmented vertebral column, or backbone, which replaces the embryonic notochord in the adult form. This backbone is composed of individual bones or cartilage blocks called vertebrae, which interlock to protect the dorsal hollow nerve cord (the spinal cord).
The internal skeleton (endoskeleton) of vertebrates is also characterized by a cranium, a bony or cartilaginous structure that encloses and protects the brain. This combination of a vertebral column and a cranium allowed for the evolution of larger, more complex bodies and sophisticated nervous systems. The vast majority of familiar animals, including bony fishes, amphibians, reptiles, birds, and mammals, belong to this subphylum.
This advanced skeletal structure provides a robust framework for muscle attachment, enabling greater size and mobility than is possible for invertebrate chordates. The vertebrate skeleton can be made of either cartilage (as seen in sharks and rays) or bone, which is characteristic of the majority of terrestrial and aquatic species.
The Chordates That Lack Backbones
The Phylum Chordata includes two subphyla of marine invertebrates that retain the four chordate characteristics but never form a vertebral column. These are the Subphylum Cephalochordata (lancelets or amphioxus) and the Subphylum Urochordata (tunicates or sea squirts). Lancelets are small, fish-like organisms, typically only a few centimeters long, that spend most of their lives buried in the sand of warm temperate and tropical seas.
Adult lancelets are unique because they retain all four defining characteristics throughout their entire lives, using their persistent notochord for axial support. They are filter feeders, drawing in water through the mouth and filtering food particles through numerous pharyngeal slits. Tunicates, conversely, are highly modified as adults, often appearing as sessile, sac-like organisms attached to the ocean floor.
The larval stage of the tunicate is mobile and possesses all four chordate traits, including a notochord that extends only into the tail region. During metamorphosis into the adult form, the larva settles, and the notochord, dorsal nerve cord, and tail are reabsorbed. The sessile adult retains only the pharyngeal slits, which are used for suspension feeding. This demonstrates a life cycle where the defining chordate features are temporary.