Taxonomy organizes the biological world into a nested hierarchy of groups, classifying organisms based on shared characteristics and evolutionary relationships. Chordata represents one of these major groupings, containing all animals that share a particular set of defining physical traits during some stage of their life cycle. This group includes many familiar animals, from humans to fish, and understanding its classification level helps appreciate the biological unity within this diverse assembly.
The Definitive Classification Rank
Chordata is classified at the taxonomic rank of Phylum, a major division directly beneath the Kingdom Animalia in the standard Linnaean hierarchy. The phylum level represents a broad grouping of organisms that share a fundamental body plan, making it one of the most inclusive ranks after the kingdom. For instance, the Phylum groups a cat with fish and birds, while the subsequent Class Mammalia narrows the focus. Phyla are defined by these basic structural blueprints, setting Chordata apart from other animal phyla like Arthropoda (insects and spiders) or Mollusca (snails and clams).
Defining Features of Chordata
The designation of Phylum Chordata is based on four specific morphological characteristics that must be present at some point during an organism’s development, even if only in the embryonic stage.
The first feature is the notochord, a flexible, rod-shaped structure that provides skeletal support along the body’s length. In most chordates, this notochord is later replaced by a bony vertebral column, which is how vertebrates get their name.
Another defining trait is the dorsal hollow nerve cord, which is positioned above the notochord. This nerve cord develops into the central nervous system, including the brain and spinal cord, in most chordates.
A third characteristic is the presence of pharyngeal slits, a series of openings located in the pharynx. In aquatic chordates, these slits develop into gill supports, but in terrestrial animals, they are often only visible during embryonic development.
Finally, all chordates possess a post-anal tail, a muscular extension of the body that continues past the anus. While used for propulsion in aquatic species, it is significantly reduced or absent in the adult form of some terrestrial chordates, such as humans. The temporary or permanent presence of all four features biologically unites every member of the Phylum Chordata.
Major Groups within the Phylum
The Phylum Chordata is divided into three main subphyla, demonstrating the wide range of diversity within the group. The most familiar subphylum is Vertebrata, which includes all animals that possess a backbone (mammals, birds, reptiles, amphibians, and fish). Vertebrates are the most structurally complex, but they account for only a small fraction of the total number of chordate species. The other two subphyla, Urochordata and Cephalochordata, consist of invertebrate chordates, meaning they do not develop a vertebral column.
Urochordata and Cephalochordata
Urochordata, commonly known as tunicates or sea squirts, are sessile marine filter feeders in their adult stage. Their chordate characteristics, including the notochord and nerve cord, are typically present only in the short-lived, free-swimming larval stage.
The final subphylum, Cephalochordata, includes the lancelets, which are small, fish-shaped marine creatures that bury themselves in the sand. Lancelets are unique among the chordates because they retain all four defining characteristics throughout their entire adult life.