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The major subphylum of the phylum Chordata, comprising the backboned animals, including humans. The subphylum is sometimes called the Craniata, because of the common possession of a cranium or braincase, but that term has dropped out of use in scientific nomenclature.
The characteristic features of the Vertebrata are a vertebral column, or backbone, and a cranium, which protects the central nervous system (brain and spinal cord) and major sense organs; the presence of bone, which is a tissue unique to vertebrates; and a neural crest of nerve cells that remain after the formation of the central nervous system. Other distinctive vertebrate features are a kidney, with the nephron as its functional unit; a heart; red and white blood cells; a liver and a pancreas; specialized sense organs, such as a complex eye, a lateral-line system, ears, and a sense of smell; several unique endocrine organs, such as the pituitary and thyroid; and a complex skin comprising an epidermis and dermis. See Chordata
Vertebrates evolved from a lower chordate similar to the present-day Cephalochordata (amphioxus). They originated in fresh water and developed a kidney as their organ of water balance. They became free-swimming, with several evolutionary lines invading the oceans. The main line of evolution in the vertebrates, that which led to the tetrapods, remained in fresh waters.
The Vertebrata are divided into the following eight classes, which are arranged into several partly overlapping informal groups, and often two superclasses, Pisces and Tetrapoda, are used to differentiate the aquatic and the terrestrial vertebrates. See Pisces (zoology)
- Superclass Pisces
- Class: Agnatha
- Superclass Tetrapoda
- Class: Amphibia
The term Gnathostomata designates the seven classes of jawed vertebrates, in contrast to the jawless Agnatha. The Anamniota include the Pisces (fishes) and the Amphibia (amphibians). The Amniota consists of the Reptilia, Aves, and Mammalia. See Amniota, Animal kingdom
(Craniota), a subphylum of the phylum Chor-data; the most highly organized and varied group of animals and the most advanced branch of Deuterostomia.
There are far fewer species of vertebrates than of invertebrates —40,000 or, according to some sources, about 45,000 vertebrate species compared to about 1.25 or 1.45 million invertebrate species. However, vertebrates are more important in the life of the modern biosphere because they usually complete the food chains in biocenoses. Vertebrates have the most varied adaptive types and life forms. Such major taxonomic categories as cyclostomes, fishes, amphibians, reptiles, birds, and mammals differ sharply in their adaptive characteristics; and even within these groups there is a great variety of adaptive types and life forms. This results both from the high level of development and complexity of vertebrates and from their great versatility in adapting to the most varied conditions of existence, from the depths of oceans to mountaintops and arid deserts.
The evolutionary process in all vertebrates involved development from a single basic structure. This development took the form of improvements on the basic structure, which affords great potentialities for modification. (A sessile, or attached, mode of existence and parasitism were of no significance in vertebrate evolution, whereas in other groups these factors led to the disruption of the line of development and to morphological regression.) Development along this basic line brought into being forms highly organized in terms of morphological structure and such biochemical and physiological properties as constant body temperature and intrauterine development of embryos. This development also gave rise to the highest forms of individual and group behavior and mental activity.
The ancestors of vertebrates, the lower chordates (Tunicata and Acrania), lived in the sea. Vertebrates originated in fresh water, where they went through the initial stages of evolution. The primary axial skeleton, the chorda, was replaced by a spinal column consisting of a series of movable articulated cartilaginous vertebrae in Agnatha and in some fishes or of bony vertebrae in other vertebrates. This resulted in the formation of a strong, flexible support for the powerful muscles needed to keep vertebrates from being carried away by currents of water. Such intensive motor activity required improvement of digestion, respiration, excretion, and blood circulation, as well as of the sense organs and central nervous system. Improvement became especially necessary after vertebrates emerged onto land and, instead of moving in water in a state of relative weightlessness, now moved on land by means of jointed (lever) extremities, or legs. A special conveyer system of digestive enzymes acting successively on food developed in such parts of the lengthened digestive tract as the oral cavity, esophagus, stomach, and intestine. The liver, a complex chemical laboratory, came to perform a variety of functions. Other developments were the powerful heart muscle consisting of several main sections, including the auricles and ventricles, and such auxiliary sections as the sinus venosus and arterial cone, and a closed circulatory system. The respiratory organs of such aquatic vertebrates as Agnatha and fishes are gills, which permit a highly efficient exchange of gases between the surrounding water and the organism’s blood. Terrestrial vertebrates developed new respiratory organs— paired lungs. When the ancestors of vertebrates began living in fresh water, they developed new organs for excretion and for water and salt metabolism: truncal, or mesonephric, kidneys (the mesonephros; in larvae, the pronephros). These organs remove excess water entering fishes’ bodies in large quantities through penetrable integuments. The mesonepheric kidneys in amniotes gave way to metanephric, or pelvic, kidneys (the metanephros) capable of making the most economical use of water when excreting metabolic products. Hormonal regulation of metabolism by the endocrine glands has become very complex in vertebrates as compared to invertebrates.
The nervous system of vertebrates consists of a brain and a spinal cord. This system, unlike that of invertebrates, has a tubular structure and consists of a vast number of nerve cells, or neurons, that form various functional complexes (the brain’s gray matter) connected by a complex system of nerve fibers (the brain’s white matter). The brain is far more complex than the central nervous system of invertebrates. The increased mobility and activity and the complex behavior of vertebrates are accompanied by improvement in the structure and functions of the sense organs. Vertebrates use many channels to obtain information about the environment. They have image vision, high acoustic capacity (hearing), refined senses of taste and smell, complex mechanoreceptors, including a system of lateral organs in Agnatha and fishes, and thermoreceptors, including the special heat locators of pit vipers. Such vertebrates as electric fish have electric and magnetic sensitivity.
Vertebrates are usually distinct sexually. The sexual organs— ovaries and testes— are generally paired, but fishes are often hermaphrodites. The lower vertebrates are usually oviparous. All groups except cyclostomes and birds are viviparous, and viviparity is the chief form of reproduction in mammals. Such higher vertebrates as birds and mammals display concern for their young. Signal heredity, or the transmitting of parental experience from generation to generation by means of instruction, is typical of most higher mammals.
The first vertebrates appeared during the Ordovician period; their remains have been found in the sediments of freshwater lakes and streams. The acquisition of a highly efficient motor system and improvement of metabolism enabled vertebrates to disperse widely and settle in the sea, where they competed successfully with the sea’s indigenous inhabitants, especially the crustaceans and cephalopod mollusks. Life in all types of bodies of water, including those with insuffcient oxygen, prepared the vertebrates to emerge onto land; the Devonian Ichthyostega that originated from the Crossopterygii apparently did so. The reptiles dominant in the Mesozoic period gave way to mammals in the Triassic period and to birds in the Jurassic period.
There are six classes of modern vertebrates: Cyclostomata; Pisces, which are often divided into two classes, Chondrichthyes and Osteichthyes; Amphibia, Reptilia, Aves, and Mammalia. These classes may also be combined into superclasses or other groups. The Cyclostomata, like the Agnatha, contrast with the remaining vertebrates, the Gnathostomata. The Anamnia (cyclostomes, fishes and amphibians), which lack complex embryonic membranes, contrast with the Amniota (reptiles, birds, and mammals). Fishes contrast with the terrestrial vertebrates or quadrupeds (Tetrapoda).
Vertebrates are of great and varied significance for man. Most domestic and commercially raised animals are vertebrates. Many mammals, birds, reptiles, amphibians, and fish do harm to the economy or transmit such naturally endemic diseases as plague, tularemia, rabies, encephalitis, and rickettsiosis. Some vertebrates are poisonous.
REFERENCESBerg, L. S. Sistema ryb. Moscow-Leningrad, 1940.
Ognev, S. I. Zoologiia pozvonochnykh, 4th ed. Moscow, 1945.
Shamal’gauzen, I. I. Proiskhozhdenie nazemnykh pozvonochnykh. Moscow, 1964.
Zoologiia pozvonochnykh. Moscow, 1964.
Zhizn’zhivotnykh, vols. 4–6. Moscow, 1969-71.
Naumov, S. P. Zoologiia pozvonochnykh, 3rd ed. Moscow, 1973.
Griffin, D., and A. Novick. Zhivoi organizm. Moscow, 1973. (Translated from English.)
Romer, A. Vertebrate Paleontology, 3rd ed. Chicago-London, 1971.
V. G. GEPTNER and N. P. NAUMOV