The phylum Chordata is a major division within the animal kingdom, encompassing a diverse array of species, including fish, amphibians, reptiles, birds, and mammals. Though members range from simple marine filter-feeders to complex terrestrial organisms, they all share an evolutionary history rooted in specific physical traits. Classification into the Phylum Chordata is based on the temporary or permanent presence of four unique anatomical features. These foundational characteristics link the group’s most primitive members to its most complex.
The Four Defining Features
The single trait that gives the phylum its name is the notochord, a flexible, rod-shaped structure providing skeletal support. This rod is positioned along the dorsal side of the body, located between the digestive tract and the nerve cord. In primitive chordates, the notochord acts as the primary support structure throughout the animal’s life. In vertebrates, it is present only during embryonic development, later being replaced by the segmented vertebral column.
Running just above the notochord is the dorsal hollow nerve cord, formed from ectoderm tissue that rolls into a tube during development. This dorsal location contrasts with many other animal phyla, which typically feature solid nerve cords located ventrally. In all chordates, this nerve cord is the precursor to the central nervous system, eventually developing into the brain and spinal cord.
A third characteristic is the presence of pharyngeal slits or pouches, which are openings in the pharynx, the part of the throat just behind the mouth. In aquatic chordates like lancelets and fish, these slits are functional, allowing water to exit the body and often developing into gills for gas exchange or aiding in filter feeding. In terrestrial vertebrates, the slits are only briefly present during embryonic development, later becoming parts of the inner ear, tonsils, and other structures in the neck.
Finally, all chordates exhibit a post-anal tail, a muscular extension of the body that extends beyond the anus. In many aquatic species, this tail is a highly developed structure containing skeletal elements and muscles, making it a primary source of locomotion. While prominent in fish and many mammals, in humans and apes, this tail is reduced to a vestigial structure during embryonic development.
Chordates Without Backbones
The Phylum Chordata includes two subphyla of invertebrates, animals that lack a vertebral column: Urochordata and Cephalochordata. These groups are often collectively called protochordates and represent the most ancient branches of the chordate lineage. Both groups display the four defining chordate features at some point in their life cycle, confirming their place in the phylum.
Urochordata (Tunicates)
The Subphylum Urochordata, commonly known as tunicates or sea squirts, are primarily sessile marine filter feeders in their adult form. The adult tunicate often looks like a simple sac covered by a tough outer covering called a tunic. However, the larval stage is a free-swimming, tadpole-like creature that prominently displays all four chordate features: the notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail. During metamorphosis, the larva attaches to a substrate, and the notochord, nerve cord, and tail are reabsorbed, leaving an adult that retains only the pharyngeal slits for filtering food.
Cephalochordata (Lancelets)
The Subphylum Cephalochordata, known as lancelets or amphioxus, are small, blade-shaped marine animals that spend most of their time buried in the sand. Unlike tunicates, adult lancelets retain all four chordate features throughout their entire lives. Their notochord is well-developed, extending into the head region, which provides rigid support for burrowing and swimming. Lancelets use their numerous pharyngeal slits to filter phytoplankton and other microorganisms from the water as a means of feeding.
The Vertebrate Lineage
The Subphylum Vertebrata represents the most diverse and familiar group within the phylum Chordata, containing over 65,000 species. This subphylum includes all fish, amphibians, reptiles, birds, and mammals, which are distinguished by two major evolutionary innovations. The first is the presence of a vertebral column, or backbone, a segmented series of bone or cartilage that replaces the notochord during development. This column surrounds and protects the dorsal hollow nerve cord, which develops into a highly differentiated brain and spinal cord.
The second innovation is the cranium, or skull, a protective casing of bone or cartilage that encloses the brain. This robust internal skeleton (endoskeleton) provides greater support for a larger body size and allows for more complex, active movements. The endoskeleton is built from either cartilage, as seen in sharks and rays, or a combination of bone and cartilage. The four core chordate features are retained or modified, linking vertebrates back to their simpler relatives. For example, the post-anal tail remains a source of locomotion in aquatic vertebrates, while on land, the internal skeleton allows for the evolution of paired appendages like fins, wings, or limbs.