Chordates are deuterostomes, placing them within one of the two major evolutionary branches of bilateral animals. This classification is based on fundamental characteristics established early in the embryonic development of the phylum Chordata, which includes humans, fish, and all other vertebrates. The term “deuterostome,” meaning “second mouth” in Greek, indicates a specific sequence in the formation of the digestive tract. This developmental blueprint links all chordates to a deep evolutionary past shared with other organisms.
Defining the Deuterostome Blueprint
The deuterostome classification is built upon three distinct embryological mechanisms that occur after fertilization. The most recognized characteristic involves the fate of the blastopore, the initial opening that forms during gastrulation. In deuterostomes, this opening develops into the anus, with the mouth forming secondarily at the opposite end of the embryo, defining the “second mouth” naming convention.
The first cell divisions of the zygote follow a pattern known as radial cleavage. The cleavage planes are oriented either parallel or perpendicular to the central axis, resulting in cells that are stacked symmetrically. This cleavage pattern is also indeterminate, meaning the developmental fate of each embryonic cell is not rigidly fixed early on. If an early cell is separated, it retains the potential to develop into a complete organism, allowing for regulative development.
The final defining mechanism concerns the formation of the coelom, the body cavity lined entirely by mesoderm. Deuterostomes form this cavity via a process called enterocoely. During gastrulation, the developing gut forms lateral pouches that eventually pinch off to create the mesodermal layer. These outpocketings expand and fuse to form the coelom, which houses the internal organs.
The Chordate Lineage and Its Subphyla
The Phylum Chordata is confirmed as part of the deuterostome superphylum through its adherence to the enterocoelous, radial, and indeterminate developmental pattern. All chordates are defined by four unique anatomical characteristics that appear at some stage of their life cycle. The first is the notochord, a flexible, rod-shaped structure providing skeletal support located between the digestive tube and the nerve cord.
A second defining feature is the dorsal hollow nerve cord, which develops from a plate of ectoderm rolling into a tube above the notochord. This structure is unique because it is hollow and positioned dorsally, unlike the solid, ventral nerve cords found in many other animal phyla. The third trait is pharyngeal slits, openings in the throat region, which initially allow water to exit during feeding in aquatic species. These later evolve into structures like gill supports or parts of the inner ear in terrestrial vertebrates.
The final feature is a post-anal tail, a posterior extension of the body that continues past the anus, containing skeletal elements and muscles. The phylum Chordata is divided into three subphyla: Vertebrata, Cephalochordata, and Urochordata. Vertebrates are the most familiar group, where the notochord is typically replaced by the vertebral column during development.
Cephalochordates (lancelets) are small, fish-like marine organisms that retain all four chordate characteristics throughout their adult lives. Urochordates (tunicates), such as sea squirts, often display the defining chordate features only in their free-swimming larval stage. They lose the notochord and dorsal nerve cord as they metamorphose into sessile adults. Despite the vast diversity in adult forms, the underlying deuterostome developmental processes unite all members of the phylum Chordata.
Placing Chordates in the Animal Kingdom
The placement of chordates within the Deuterostomia superphylum highlights a fundamental split in the evolution of bilaterally symmetrical animals. The other major phyla that share this lineage are Echinodermata (sea stars and sea urchins) and Hemichordata (acorn worms). These three phyla form the core of the deuterostome clade, sharing the same radial, indeterminate cleavage and enterocoelous coelom formation.
This superphylum stands in direct contrast to the Protostomia, the other major lineage of bilaterian animals, which includes phyla like Arthropoda, Mollusca, and Annelida. Protostomes follow a different developmental trajectory, characterized by spiral cleavage, where the cell divisions are diagonal to the main axis of the embryo. Their cleavage is determinate, meaning the fate of the early cells is fixed, and separating them results in incomplete development.
The protostome name, meaning “first mouth,” reflects their pattern where the blastopore forms the mouth, and the anus develops later. The coelom in protostomes is also formed differently, through schizocoely, which involves the splitting of solid masses of mesoderm. This deep evolutionary division represents two distinct solutions to building a complex, bilaterally symmetrical body.