Chordata

Human beings are chordates—of the phylum chordata—and so are all other vertebrates, or animals with a spinal column. In addition, there are two invertebrate groups of chordates: the urochordates and the cephalochordates.

The Urochordata (e.g., tunicates) and Cephalochordata (e.g., lancelets) were the earliest chordates to evolve, and they provide a link between invertebrate and vertebrate animals. However, as different as these organisms are from each other and from vertebrate chordates, they all share the following characteristics that identify them as chordates (and distinguish them from all other invertebrate animals): a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits. Many adult vertebrates have no notochord or pharyngeal gill slits, but these structures can nevertheless be found in their embryos.

The notochord is a long, elastic rod that provides structural support to the chordate body. In cephalochordates it prevents the body from shortening when muscle fibers in the body wall draw together, causing a bending from side to side and propulsion of the animal. In most vertebrates (except some fishes), bony vertebrae develop around the nerve cord and the noto-chord, and the vertebral structures largely replace the notochord in most adult vertebrates. However, some adult vertebrates may retain remnants of the notochord (e.g., the gelatinous disks between the vertebrae of humans). The dorsal hollow nerve cord is a key element of the chordate nervous system and is present in all chordates. In vertebrate embryos it develops into the spinal cord and the brain.

The pharyngeal pouches with gill slits originally evolved as filter-feeding devices and can still be found as such in invertebrate chordates. During some point in their development all chordates still exhibit them. However, among the vertebrates only fish retain pharyngeal gill slits as adults. The cartilage-based rods that support the gill bars (the solid areas between the gill slits) in invertebrate chordates gave rise to the vertebrate jaw during vertebrate evolution, completely changing the feeding method in this group of animals. Subsequently, some of the bones in the vertebrate jaw evolved into middle-ear bones in amphibians, reptiles, birds, and mammals; these bones assisted in the transmission of sound and hearing when early vertebrates moved from life in the water onto land.

Vertebrates differ greatly from other chordates in size and activity level, and the evolution of their distinctive characteristics is largely correlated with this difference. Vertebrates actively move around looking for food. This led to the concentration of sense organs at the front end of the body and an accumulation of nerve cells (i.e., a brain) to process all the sensory information. The need for more efficient movement led to the evolution of a stronger support system (vertebral column), a bony skeleton, and four limbs to support the body on land.

Today the vertebrates, with nearly 43,000 living species, are the most diverse group of all chordates. All vertebrate species can be grouped into seven different classes: Agnatha (jawless fishes), Chondrichthyes (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia (amphibians), Reptilia (reptiles), Aves (birds), and Mammalia (mammals).

see also Phylogenetic Relationships of Major Groups.

Kathrin F. Stanger-Hall

Bibliography

Alexander, R. McNeill. The Chordates. New York: Cambridge University Press, 1975.

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, Inc., 1990.

Kluge, Arnold G. Chordate Structure and Function, 2nd ed. New York: Macmillan Publishing, 1977.

Walker, Warren F. Functional Anatomy of the Vertebrates: An Evolutionary Perspective. Philadelphia: Saunders College Publishing, 1987.

The first chordates appear as fossils in rocks from the Cambrian period. These rocks are approximately 570 million years old.

Chordata

Chordata is a large and diverse group of animals, with roughly 50,000 living species included. The majority of chordates belong to a group called Vertebrata. Vertebrates have backbones that are composed of vertebrae. Some examples of vertebrates are sharks, fish, dinosaurs, and human beings.

A second group of chordates, called Urochordata, consists of animals found mostly in oceans. Urochordates include sock-shaped pyrosomes that grow up to 10 meters (32.8 feet) long, sack-shaped sea squirts that live attached to the seafloor, and tadpole-shaped larvaceans that build their floating houses out of mucus.

All of these assorted chordates are united because they are descended from a common ancestor that had three features that were passed on to all of its descendants. These three characteristics can be used to distinguish chordates from other animals.

First, chordates have a collection of nerve fibers, called a nerve cord, which runs down their back sides connecting the brain to the organs and muscles. The second characteristic is a notochord, which is a stiffened rod that runs underneath the nerve cord. The notochord is used by many chordates as an aid for swimming. Muscles pull the notochord one way and then it springs back, propelling the chordate forward through the water.

Finally, all chordates have pharyngeal slits, a set of openings behind the head that connect directly to the throat. Some chordates use their pharyngeal slits to filter food out of water sucked in through their mouths. Other chordates have modified pharyngeal slits, called gills, that are used to get oxygen out of water. Human beings, like other land-dwelling chordates, only have pharyngeal slits as an embryo. During a baby's development they are modified into parts of the inner ear.

see also Animalia; Taxonomy

Allen G. Collins

Bibliography

Newman, H. H. The Phylum Chordata: Biology of Vertebrates and Their Kin. New York: Macmillan, 1939.

Ruppert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Fort Worth, TX: Saunders College Publishing, 1996.

Weichert, Charles K. Elements of Chordate Anatomy, 3rd ed. New York: McGraw-Hill,1967.

Chordata A phylum of animals characterized by a hollow dorsal nerve cord and, at some stage in their development, a flexible skeletal rod (the notochord) and gill slits opening from the pharynx. There are four subphyla: the Urochordata (sea squirts), Cephalochordata (lancelets), Agnatha (jawless chordates), and Gnathostomata (jawed chordates). In the Agnatha and Gnathostomata, commonly known as vertebrates or craniates, the notochord is present only in the embryo or larva and becomes replaced by the vertebral column (backbone) before birth or metamorphosis. This has permitted the vertebrates a greater degree of movement and subsequent improvement in the sense organs and enlargement of the brain, which is enclosed in a skeletal case, the cranium.

In some classifications the two nonvertebrate subphyla are elevated to the status of phyla and the jawed and jawless chordates are included together in a third phylum, Craniata, containing a single subphylum, Vertebrata. The old subphyla Agnatha and Gnathostomata are then regarded as superclasses of the Vertebrata.

Chordata (chordates) Large phylum comprising the animals that possess a rod of flexible tissue (notochord), which is protected in higher forms by a vertebral column (see VERTEBRA). The phylum includes the Craniata, the Urochordata, and the Cephalochordata. The first chordates and the earliest vertebrates (Craniata) are both found in Cambrian rocks, See PIKAIA.

Chordata (kingdom Animalia) A large phylum comprising the animals that possess a rod of flexible tissue (notochord), which is protected in higher forms by a vertebral column. The phylum includes Urochordata (tunicates), Amphioxus, fish, amphibians, reptiles, birds, and mammals. The first chordates and the earliest vertebrates are both found in Cambrian rocks.