The animal kingdom is divided into major groupings based on differences in embryonic development. One of the largest is the superphylum Deuterostomia, which includes many familiar life forms. The name Deuterostomia translates from Greek to “second mouth,” describing a defining characteristic of these organisms. This classification helps scientists understand the deep evolutionary relationships between diverse animal groups.
The Defining Characteristic of Deuterostomes
The primary feature uniting all deuterostomes is the specific sequence of events during early embryonic development. This process begins with the blastopore, the first opening that forms in the embryo. In deuterostomes, the blastopore ultimately develops into the anus, while the mouth forms later from a secondary opening.
The early cell division, known as cleavage, also follows a distinct pattern called radial cleavage, where the cell divisions are either parallel or perpendicular to the central axis. This contrasts with the spiral cleavage seen in other animal groups. Furthermore, this cleavage is indeterminate, meaning the developmental fate of each embryonic cell is not set in stone early on. If the cells are separated at an early stage, each has the potential to develop into a complete organism.
The formation of the coelom, or true body cavity, also distinguishes deuterostomes. This process is called enterocoely, occurring when the mesoderm develops from outpocketings of the archenteron, or primitive gut. These pouches pinch off from the endodermal lining to form the mesodermal layer, which then fuses to create the coelomic cavity. These three developmental traits form the foundation of the Deuterostomia superphylum.
Phylum Chordata
Phylum Chordata is the largest and most well-known group of deuterostomes, containing vertebrates and two groups of invertebrates. All members possess four specific traits at some point during their life cycle, though they may not be present in the adult form. The first is a stiff, flexible rod called the notochord, which provides skeletal support.
Chordates possess a dorsal hollow nerve cord situated above the notochord, which develops into the brain and spinal cord in complex species. Pharyngeal slits or clefts, which are openings in the pharynx, function as filter-feeding structures in some aquatic species. The fourth characteristic is a post-anal tail, a muscular extension of the body that continues past the anus.
The Phylum Chordata is divided into three major subphyla. Subphylum Vertebrata includes all animals with a backbone, where the notochord is replaced by a vertebral column during development. Subphylum Urochordata, or tunicates (sea squirts), are marine filter feeders that only possess chordate traits during their larval stage. Subphylum Cephalochordata, or lancelets, are small, fish-like marine organisms that retain the notochord throughout their lives.
Phylum Echinodermata
The second major phylum is Echinodermata, which exclusively contains marine animals like sea stars, sea urchins, and sea cucumbers. While their larvae exhibit bilateral symmetry, adult forms are characterized by pentaradial symmetry, a body plan arranged in five parts around a central axis. This unique radial arrangement is evident in the five arms of a sea star or the five-part body structure of a sea urchin.
Echinoderms are defined by their specialized water vascular system, a network of fluid-filled canals derived from the coelom. This system opens to the outside through a perforated plate called the madreporite. The water vascular system powers the tube feet, which are used for locomotion, feeding, gas exchange, and waste removal. The movement of water through the system creates hydrostatic pressure, allowing the tube feet to extend or retract for movement.
Other Deuterostome Phyla
Beyond the two most populous phyla, the Deuterostomia superphylum also includes Phylum Hemichordata. These marine invertebrates, including acorn worms and pterobranchs, share a close evolutionary relationship with echinoderms. Acorn worms are solitary, burrowing organisms that have a distinct three-part body plan and possess pharyngeal slits similar to those found in chordates.
Hemichordates and echinoderms are often grouped together into a clade known as Ambulacraria, highlighting their shared ancestry and developmental features. This grouping distinguishes them from Phylum Chordata. A small, enigmatic group of marine worms called Phylum Xenoturbellida has also been recognized as a separate deuterostome lineage, sometimes considered basal to the Ambulacraria. These simple, sac-like organisms complete the list of extant phyla classified within the superphylum Deuterostomia.