See G. G. Luce, Biological Rhythms in Human and Animal Physiology (1971); J. Brady, ed., Biological Timekeeping (1982); L. Glass and M. C. Mackey, From Clocks to Chaos: The Rhythms of Life (1988).
In spherical, or point, symmetry, any straight cut through the central point of a sphere divides it into mirror-image halves. Point symmetry, often called universal symmetry by biologists, is seen in some floating animals with radiating parts, such as the single-celled protozoans of the order Radiolaria.
Radial, or line, symmetry, as exemplified by a cone or a disk that is symmetrical about a central axis, is especially suitable for sessile or floating animals. Most radially symmetrical animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. Radial symmetry is seen in sessile organisms such as the sea anemone, floating organisms such as jellyfish, and slow-moving organisms such as sea stars, or starfish. Many jellyfish have four radial canals and are said to have tetramerous radial symmetry; sea stars, with five arms, have pentamerous radial symmetry. Many flowers, such as dandelions and daffodils, are radially symmetrical. Nonradial parts, such as the slit-shaped gullets of sea anemones, are often present in otherwise radial animals.
In plane, or bilateral, symmetry, one particular plane, termed the sagittal plane, divides the body into two equal halves, usually right and left halves that are mirror images of each other. Flowers such as orchids and sweet peas are bilaterally symmetrical. Bilateral symmetry is most suitable for actively moving organisms, as it permits streamlining, and is the most common symmetry among animals. In animals this symmetry type also favors the formation of main nerve centers and special sense organs and contributes to cephalization, or the evolutionary development of a head.
Periodic biological fluctuation in an organism corresponding to and in response to periodic environmental change, such as day and night or high and low tide. The internal mechanism that maintains this rhythm even without the apparent environmental stimulus is a “biological clock.” When the rhythm is interrupted, the clock's adjustment is delayed, accounting for such phenomena as jet lag when traveling across time zones. Rhythms may have 24-hour (circadian rhythm), monthly, or annual cycles. Seealso photoperiodism.
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Gradual changes in size, shape, and function during an organism's life that translate its genetic potentials (genotype) into functioning mature systems (phenotype). It includes growth but not repetitive chemical changes (metabolism) or changes over more than one lifetime (evolution). DNA directs the development of a fertilized egg so that cells become specialized structures that carry out specific functions. In humans, development progresses through the embryo and fetus stages before birth and continues during childhood. Other mammals follow a similar course. Amphibians and insects go through distinctive stages that are quite different. In plants, the basic pattern is determined by the arrangement of lateral buds around a central growing stem. Different rates of growth of the plant's component elements then determine its shape and that of various parts. In both animals and plants, growth is greatly influenced by hormones; factors within individual cells probably also play a role.
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Military use of disease-producing or poisonous agents, and the means for defending against such agents. Biological warfare agents include many bacteria, such as those which cause anthrax, brucellosis, and typhus; viruses that cause diseases such as equine encephalitis; fungi such as rice blast, cereal rust, wheat smut, and potato blight; and toxins such as botulinum and ricin that are extracted from living organisms. Biological warfare dates from ancient times when warring groups would try to poison enemy soldiers with rotting or diseased corpses, infect cattle and horses, or spread contagion through civilian populations. Following the horrors of World War I, a 1925 Geneva Protocol prohibited the use of biological agents in warfare; however, this did not prevent Japan from using them in China during World War II. During the Cold War the Soviet Union as well as the U.S. and its allies built huge stockpiles of biological agents. Both sides signed the 1972 Biological Weapons Convention, which prohibits the production, stockpiling, or development of biological weapons and requires the destruction of existing stockpiles, but the Soviets conducted a clandestine program until the 1990s. Biological weapons programs can be concealed easily, and the 1972 convention contains no provisions for inspection and reporting. As a result, many states have been suspected of developing biological warfare agents, and some modern armed forces have prepared defensive measures. These include battlefield sensors, protective garments and masks, sterilizing agents, and vaccines.
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Study of the physiological basis of behaviour. Traditional specializations in the field cover perception, motivation, emotion, learning, memory, cognition, or mental disorders. Also considered are other physical factors that affect the nervous system, including heredity, metabolism, hormones, disease, drug ingestion, and diet. An experimental science, physiological psychology relies heavily on laboratory research and quantitative data.
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In biology, the thin layer that forms the outer boundary of a living cell or of an internal cell compartment. The outer boundary is the plasma membrane, and the compartments enclosed by internal membranes are called organelles. Biological membranes have a dual function: separation of vital but incompatible metabolic processes conducted in the organelles; and passage of nutrients, wastes, and metabolic products between organelles and between the cell and the outside environment. Membranes consist largely of a double layer of lipids in which are embedded large proteins, many of which transport ions and water-soluble molecules across the membrane. Seealso cytoplasm, eukaryote.
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Quantity of plant and animal species found in a given environment. Sometimes habitat diversity (the variety of places where organisms live) and genetic diversity (the variety of traits expressed within a species) are also considered types of biodiversity. The estimated 3–30 million species on Earth are divided unequally among the world's habitats, with 50–90percnt of the world's species living in tropical regions. The more diverse a habitat, the better chance it has of surviving a change or threat to it, because it is more likely to be able to make a balancing adjustment. Habitats with little biodiversity (e.g., Arctic tundra) are more vulnerable to change. The 1992 Earth Summit resulted in a treaty for the preservation of biodiversity.
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Materials and substances: