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from article Rotifera

Four of the 8 or 9 recognized types of rotifer mastax. (a) Forcipate mastax of the monogonont Albertia vermiculus has all parts of the mallei and incus. (b) Incudate mastax of the monogonont Asplanchna brightwellii has enlarged, pincerlike rami and…

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from article Rotifera

Life cycles of the three rotifer groups. (a) Gonochoristic reproduction in seisonids. (b) Parthenogenesis alternating with sexual reproduction in monogononts. (c) Obligate parthenogenesis in bdelloids. 2n represents diploidy, and n represents…

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from article Fat and oil (food)

Olive oil and other sources of healthy fats in food. Cocoa butter in dark chocolate is a mixture of fatty acids. (Credit: Lumus Photography/iStock/Getty Images)

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from article Fat and oil (food)

Refining process for fats and oils. RBD = refined, bleached, and deodorized.

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from article Apatite

Apatite. (a) Specimen from Snarum, Norway. (b) Common crystal habits.

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from article Andalusite

Andalusite crystals in metamorphic rock. (Credit: RF Company/Alamy)

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from article Andalusite

Phase diagram for the polymorphs of aluminum silicate (Al2SiO5). 1 km = 0.6 mi; 1 kb = 102 MPa.

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from article Archaeopteryx

Cast of the Archaeopteryx fossil specimen located at the Berlin Natural History Museum. Notable features of this fossil bird include spread-out wings and long clawed fingers. (Copyright © Natural History Museum, London)

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from article Extraterrestrial intelligence

A portion of the Allen Telescope Array used by the SETI Institute in its search for extraterrestrial intelligence. (Credit: Seth Shostak/SETI Institute)

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from article Extraterrestrial intelligence

The Green Bank Telescope, which has a collecting dish area that covers more than 9300 m2. (Credit: NRAO/AUI/NSF)

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from article Extraterrestrial intelligence

The traditional Drake equation, appearing in full in the top row with its terms explained, offers a rough way to calculate the number of technologically advanced civilizations in our home galaxy, the Milky Way. The bottom row offers a variation on…

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from article Calcarea

A calcareous sponge belonging to the family Leucettidae (order Clathrinida). (Credit: Bernard Dupont)

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from article Calcarea

Grades of construction of the water-current system in Calcarea: (a) asconoid type; (b) syconoid type without cortex; (c) syconoid type with cortex; (d) intermediate or transitional stage between syconoid and leuconoid types; (e) leuconoid type.…

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from article Calcarea

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from article Anoxic zone

Water column profiles from Saanich Inlet, an intermittent anoxic fiord in British Columbia, Canada. (a) Oxygen and hydrogen sulfide profiles showing the interface at 130 m (430 ft). (b) Dissolved iron and manganese at the same location. Pump cast…

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from article Anoxic zone

Schematic representation of trends in pore water profiles. Depth and concentration are arbitrary units but represent concentrations measured or inferred (in the case of O2) in equatorial Atlantic pore waters. (After P. Froelich et al., Early…

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from article Orion Nebula

Orion Nebula and its neighborhood. (a) Visual-wavelength view of the Orion Nebula obtained by taking a time exposure on a photographic plate with the 4-m-diameter (158-in.) telescope on Kitt Peak, Arizona (National Optical Astronomy Observatories).…

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from article Orion Nebula

Closeup view of the Trapezium stars obtained with the Planetary Camera on the Hubble Space Telescope. The field-of-view shown is 35 arcseconds of each side. Most faint stars have tails pointing away from the brightest star. These low-mass stars are…

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from article Orion Nebula

Hubble Space Telescope image showing the inner portion of the Orion Nebula in the light of doubly ionized oxygen (blue), atomic hydrogen (green) and singly ionized nitrogen (red). (C. R. O'Dell and S. K. Wong, Rice University; NASA)

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from article Orion Nebula

Hubble Space Telescope image showing closeups of several disks and ionized structures surrounding low-mass stars embedded within the Orion Nebula. The stars located near the Trapezium stars exhibit bright comet-shaped regions of gas expanding away…

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from article Marine sediment

Shoal water sediments as examples of sedimentation without supply of land-derived sediment. (a) The Bahamas. (b) The Gulf of Batabano, which is in southwestern Cuba. Both of these shoals border directly on the deep ocean.

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from article Marine sediment

Sediment distribution of the northwestern Gulf of Mexico as an example of the sediments of a shelf with abundant land-derived sediment. Shelf depositional sediments and slope deposits are silty clays; nondepositional area is covered with relict…

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from article Marine sediment

Sediments on shelves with little land-derived sediment supply. (a) Off western coast of Florida. (b) Off northwestern Yucatan Peninsula.

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from article Marine sediment

Distribution of deep-sea sediments. (a) Major types. (b) Kaolinite concentration. (c) Montmorillonite concentrations. The concentrations are in the <2-μm-size fraction on a carbonate-opaline-silica-free basis.

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from article Marine sediment

Dissolved (a) oxygen (O2) and (b) nitrate (NO3−) in the interstitial water of a pelagic marine sediment. 1 cm = 0.4 in.

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from article Marine sediment

Dissolved (a) nitrate (NO3−), (b) manganese (Mn), and (c) iron (Fe) in the interstitial water from a hemipelagic marine sediment. Symbols represent different cores. 1 cm = 0.4 in.

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from article Marine sediment

Dissolved sulfate (SO42−) and methane from extremely reducing sediments in Saanich Inlet. 1 cm = 0.4 in.

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from article Marine sediment

Solubility of aragonite and calcite [expressed in terms of carbonate (CO32−) ion concentration] in seawater at 2°C (36°F) as a function of water depths in the North Atlantic and the eastern Equatorial Pacific. The intersections of the solubility…

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from article Marine sediment

Global organic carbon cycles. DOC = dissolved organic carbon; POC = particulate organic carbon. °F = (°C × 1.8) + 32; 1 m = 3.3 ft.

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from article Marine sediment

Rippled seafloor of sand being transported along the bed of a thermohaline current. From a water depth of 8900 ft (2700 m) on the northwest side of the Rockall Plateau, North Atlantic.

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from article Marine sediment

Lead-210 (210Pb) distribution in a sediment core (527-3) obtained by the research submersible Alvin from the Famous site (36°48.5′N, 33°15.3′W) on the Mid-Atlantic Ridge. The fit of the excess 210Pb [that is, unsupported by radium-226 (226Ra)]…

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from article Marine sediment

Distribution of 14C ages with depth in Famous core 527-3. The top 3.5 in. (9 cm) yield a constant age of 2400 years before present (B.P.) because of bioturbation. The deeper points define a line yielding a sediment accumulation rate of 2.9 cm/103…

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from article Marine sediment

Summary of major changes in the climate system over the past 800,000 years from a set of 18 proxy indicators. Coherent variations are evident in all components. (After J. Imbrie et al., On the structure and origin of major glaciation cycles, 1.…

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from article Invertebrate embryology

Spermatozoa and fertilizable eggs. (a) Sea urchin spermatozoon. (b) Marine annelid (Nereis) egg, with intact germinal vesicle containing a nucleolus. Spheres surrounding germinal vesicle are oil droplets. (c) Marine mussel (Mytilus) egg, germinal…

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from article Invertebrate embryology

Formation of the fertilization membrane in the sea urchin egg. (a) Surface of the unfertilized egg. (b) Explosion of the cortical granules: the vitelline membrane begins to be lifted up while the dark bodies are extruded, and the egg plasma membrane…

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from article Invertebrate embryology

Symmetry of cleavage patterns and invertebrate blastulae. (a, b) Spiral cleavage. (c) Bilateral cleavage. (d) Radial cleavage. (e) Irregular cleavage. (f) Sea urchin blastula. Relatively uniformly sized blastomeres and large blastocoele. (g) Annelid…

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from article Invertebrate embryology

Gastrulation and larval mesoderm formation. (a) Early and (b) later stage in nonyolky eggs. (c) Late blastula, (d) early gastrula, and (e) late gastrula in yolky egg of the snail. (f) Mesoderm formation in the limpet (Patella) shown in sections…

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from article Invertebrate embryology

Maturation and early cleavage in the mussel (Mytilus). (a) First polar body being formed at animal pole; polar lobe at vegetal pole. (b) First polar body completely extruded; polar lobe withdrawn. (c, d) Second polar body formation. (e–g) First…

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from article Invertebrate embryology

Stages in the development of Patella and gastrulation in a tunicate. (a) Late blastula of Patella; macromeres being covered by micromeres, and blastopore at vegetal pole. (b) Early trochophore of Patella; blastopore shifted forward. (c) Matured…

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from article Invertebrate embryology

Cleavage and gastrulation in the sea urchin. (a) Zygote. (b) Two-cell stage. (c) Four-cell stage. (d) Eight-cell stage. (e) Sixteen-cell stage. (f) Thirty-two–cell stage. (g) Vegetal region of blastula flattened; primary mesenchymal cells in…

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from article Invertebrate embryology

Larval stages. (a) Ventral and (b) side view of pluteus larva of sea urchin. (c) Section through tunicate gastrula. (d) Tadpole stage of tunicate larva.

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from article Invertebrate embryology

Young echinoid Peronella japonica, after metamorphosis. (After K. Okazaki and J. C. Dan, Metamorphosis of partial larvae of Peronella japonica Morteusen, a sand dollar, Biol. Bull., 106:83–99, 1954)

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from article Invertebrate embryology

Metamorphosis in tunicate. (a) Beginning of metamorphosis. (b) Young adult.

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from article Peritrichia

Peritrichida. (a) Vorticella, a stalked peritrich. (b) Trichodina, a mobile peritrich.

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from article Gravitational radiation

Volume rendering of the gravitational radiation generated during the merger of a binary black hole system. The red spheres represent the horizons of the black holes. The color map shows positive wave amplitudes in yellow/red and negative amplitudes…

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from article Gravitational radiation

Effect of a gravitational wave, passing in a direction perpendicular to the plane of the paper, on a ring of test particles. The ring is distorted into an ellipse, elongated in one direction in one half-cycle of the wave, and elongated in the…

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from article Gravitational radiation

Orbital decay of the binary pulsar PSR 1913+16, discovered by Russell Hulse and Joseph Taylor in 1974. Data points show the measured change in the orbit (the periastron is the minimum separation of the pulsar from the center of mass of the binary…

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from article Gravitational radiation

The gravitational-wave signal GW150914, recorded by the LIGO detectors at Hanford, Washington, and Livingston, Louisiana, on September 14, 2015. The thick orange and blue lines show the data from the Hanford and Livingston detectors, respectively,…

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from article Ecological succession

A plant succession involves a predictable set of vegetative changes through a series of successional stages. (U.S. Environmental Protection Agency)

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from article Ecological succession

A spruce tree growing among a stand of cottonwoods. Spruce trees may eventually become the dominant species in this ecosystem. (U.S. National Park Service/Kay White)

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from article Schizophrenia risk genes

Genomic distribution of schizophrenia-associated variants. Represented by red dots, these variants signpost the genomic neighborhoods of schizophrenia risk genes on the 24 human chromosomes. Clusters of association signals (“hot spots”) are visible.…

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from article Oceanography

Oceanographic buoy stationed in the Aleutian Islands area of North Pacific Ocean. (Credit: NOAA Office of Marine and Aircraft Operations)

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from article Vesuvianite

Vesuvianite. (a) Crystal, Christiansand, Norway (specimen from Department of Geology, Bryn Mawr College). (b) Crystal habits (after C. Klein and C.S. Hurlbut, Jr., Manual of Mineralogy, 21st ed., 1993)

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from article Bacterial growth

Colonial growth displayed by Gram-negative Yersinia pestis bacteria, which were grown on a medium of chocolate agar, for a 72-h time period, at a temperature of 25°C (77°F). Yersinia pestis is pathogenic toward humans and is responsible for causing…

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from article Bacterial growth

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from article Indian Ocean

Physiographic features of the Indian Ocean.

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from article Indian Ocean

Winds over the Indian Ocean.

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from article Indian Ocean

Surface currents of the Indian Ocean.

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from article Indian Ocean

Sea-surface temperature (°C) of Indian Ocean.

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from article Indian Ocean

Sea-surface salinity of the Indian Ocean in parts per thousand by weight (‰ ).

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from article Indian Ocean

Indian Ocean from the Arabian Sea to Antarctica showing (a) temperature (°C), (b) salinity (‰), (c) oxygen content (ml/liter), and (d) water masses.

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from article Indian Ocean

Mid-ocean ridge system of the Indian Ocean. DSDP = Deep Sea Drilling Project.

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from article Indian Ocean

Geotectonic image of the Indian Ocean as reflected in the satellite-derived gravity field of the region. A few specific structural features are identified as follows: Spreading centers: A, Carlsberg Ridge; B, Southwest Indian Ridge; C, Southeast…

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from article Sex chromosome

Schematic of the human sex chromosomes. Males have an X chromosome and a Y chromosome, whereas females have two X chromosomes. On the Y chromosome, it is the SRY (sex-determining region on the Y) gene that triggers male development. (Copyright ©…

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from article Andreaeopsida

Granite moss (Andreaea ruperstris), showing gametophyte with sporophytes. (After G. M. Smith, Cryptogamic Botany, vol. 2, 2d ed., McGraw-Hill, 1955)

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from article Porphyroblast

Porphyroblasts. (a) Porphyroblastic mica schist. Large crystals (porphyroblasts) of biotite mica formed late and replaced the rock except for the enclosed grains (quartz) aligned parallel to foliation. Smaller porphyroblasts of garnet replaced the…

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from article Priapulida

Priapulida. (a) Priapulus adult and (b) larva.

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from article Rock varnish

Varnished river cobble in western China.

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from article Rock varnish

Microlaminations seen in a varnish ultrathin section under a light microscope. Maximum varnish thickness is 150 μm.

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from article Insect physiology

Brood X cicada drying out after shedding its exoskeleton. (Credit: Michael J. Shirey)

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from article Insect physiology

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from article Insect physiology

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from article Hydrology

Diagram of the hydrologic cycle.

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from article Hydrology

Water flow scheme of the hydrologic cycle.

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from article Spearmint

Native spearmint (Mentha spicata). (Credit: Gerald D. Carr)

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from article Severe acute respiratory syndrome (SARS)

Electron microscopic image of a thin section of a severe acute respiratory syndrome coronavirus (SARS-CoV) within the cytoplasm of an infected cell, showing the spherical particles and cross sections through the viral nucleocapsid. (Credit: C. S.…

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from article Blueberry

Blueberries (Vaccinium sp.). (Photograph courtesy of Scott Bauer, Agricultural Research Service, United States Department of Agriculture)

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from article Blueberry

Mummy berry disease caused by the fungus Monilinia vaccinii-corymbosi. (a) Healthy fruit and (b) diseased fruit (from R. D. Milholland, Sclerotium germination and histopathology of Monilinia vaccinii-corymbosi on highbush blueberry, Phytopathology,…

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from article Botulism

A colony of Clostridium botulinum, the causative agent of botulism, growing on egg yolk agar. (Credit: Larry Stauffer/Centers for Disease Control and Prevention)

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from article Ribonucleic acid (RNA)

Structure of ribonucleic acid (RNA). Each nucleotide is composed of three parts: a five-carbon sugar known as ribose; a phosphate group (P); and one of four bases attached to each ribose. The four bases are adenine (A), cytosine (C), guanine (G),…

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from article Ribonucleic acid (RNA)

Structural representations of a generalized tRNA molecule. (a) Secondary structure model showing base-pairing interactions within the tRNA. The characteristic and/or invariant nucleotides are the same regardless of the type of tRNA; the remainder…

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from article Halimeda

Halimeda kanaloana found in the waters off Hawaii. (Photo courtesy of the Deep Water Macroalgal Meadows 2004 Exploration, Office of Ocean Exploration, National Oceanic and Atmospheric Administration)

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from article Liver

The human liver is located in the right upper quadrant of the abdomen, immediately beneath the diaphragm. Two hepatic lobes are recognized: a larger right lobe and a smaller left lobe. The hepatic artery and portal vein provide the liver with a dual…

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from article Liver

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from article Liver

Human liver and its major dorsal connections. Rami venae portae: C, centralis; I, intermedius; ID, inferior dexter; Inf, inferior sinister; L, lateralis; Q, quadratus; SS, superior sinister. Venae hepaticae: HCD, dorsocaudalis; HD, dextra; HM,…

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from article Liver

Liver structure. (Credit: E. Oppenheimer/Ciba)

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from article Liver

Disposition of the hepatic (liver-cell) plate and sinusoids around the terminal hepatic venule or central vein, showing centripetal flow of blood from branches of the hepatic artery and portal vein and centrifugal flow of bile to the small bile duct…

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from article Liver

Relationship of the liver (parenchymal) cells to each other and to the sinusoids, with principal components of the hepatic cell as seen in electron micrographs. (Credit: W. Bloom and D. W. Fawcett/Saunders)

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from article Stenurida

Stenurida. (a) Ventral view of an arm and mouth frame plates of Bdellacoma vermiformis, a stenurid with long, slender arms. (b) Stenaster obtusus, a stenurid with short, broad arms. (After W. K. Spencer and C. W. Wright, Asterozoans, in R. C. Moore,…

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from article Cambrian

Cambrian, the first period of the Paleozoic Era.

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from article Cambrian

Reconstruction of the Lower Cambrian world. (After W. S. McKerrow, C. R. Scotese, and M. D. Brasier, Early Cambrian continental reconstructions. J. Geol. Soc., 149:599–606, 1992)

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from article Cambrian

North American divisions of the Cambrian System. Asterisks denote levels of major trilobite extinctions.

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from article Cambrian

Representative Cambrian fossils: (a–c) trilobites; (d–f) brachiopods; (g) hyolithid; (h–i) mollusks; (j–l) echinoderms; and (m, n) archaeocyathids.

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from article Sarcopterygii

Marbled or speckle-bellied lungfish (Protopterus aethiopicus). This African lungfish, a member of the class Sarcopterygii, can reach up to 2 m (6.6 ft) in length. (Photo by Joel Abroad)

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from article Aquarius

Modern boundaries of the constellation Aquarius, the Water Bearer. The celestial equator is 0° of declination, which corresponds to celestial latitude. Right ascension corresponds to celestial longitude, with each hour of right ascension…

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from article Acoustic microscope

Acoustic lens.

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from article Acoustic microscope

Acoustic images. (a) Bipolar transistor on silicon integrated circuit and (b) myxobacterium, both images taken in superfluid helium at 0.2 K (from J. S. Foster and D. Rugar, Low-temperature acoustic microscopy, IEEE Trans., SU-32, 139–151, 1985).…

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from article Acoustic microscope