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  • egg | biology | Britannica.com
    shell membranes and the outermost membrane the shell As pointed out above this entire structure is commonly referred to as an egg Mature eggs remain functional for a relatively short period of time after which fertilization cannot occur The eggs of most invertebrates fish and amphibians must be fertilized a few minutes after they are shed into the water an exception is sea urchin eggs which are viable for about 40 hours after their release Most other animal eggs have life spans similar to that of the human egg i e 12 to 24 hours See also ovum Comments Share Email Print Cite You may also be interested in life organ cell reproduction science human body gamete human reproductive system ovum yolk hydramnios chorion Keep exploring The Human Body Fact or Fiction Human Body Fact or Fiction Human Body Fun Facts Fact or Fiction Editor Picks The 7 Best Pinecones Really Editor Picks Top 5 Most Awesome Parasitic Plants What made you want to look up egg To From Subject Comments Please limit to 900 characters Cancel Britannica Stories Behind The News Philosophy Religion Healing the Schism Pope Meets Patriarch Behind The News Science Gravitational Waves Observed Spotlight History Thomas Malthus s 250th Birthday See More Stories FEATURED QUIZZES Vocabulary Quiz True or False Spell It See More Quizzes About Us About Our Ads Contact Us Privacy Policy Terms of Use 2016 Encyclopædia Britannica Inc MLA style egg Encyclopædia Britannica Encyclopædia Britannica Online Encyclopædia Britannica Inc 2016 Web 12 Feb 2016 http www britannica com science egg biology APA style egg 2016 In Encyclopædia Britannica Retrieved from http www britannica com science egg biology Harvard style egg 2016 Encyclopædia Britannica Online Retrieved 12 February 2016 from http www britannica com science egg biology Chicago Manual of Style Encyclopædia Britannica Online

    Original URL path: http://www.britannica.com/science/egg-biology (2016-02-13)
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  • tadpole | zoology | Britannica.com
    internal gills are concealed by a covering known as an operculum Most tadpoles are vegetarians although those of a few species are carnivorous or even cannibalistic Tadpole metamorphosis follows a pattern of gradual development of forelimbs and hind limbs resorption of the tail shortening of the intestine disappearance of the gills and development of lungs Upon completion of metamorphosis the tadpole emerges onto land as a young 100 of 142

    Original URL path: http://www.britannica.com/science/tadpole (2016-02-13)
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  • Mantellidae | amphibian family | Britannica.com
    Ads Contact Us Privacy Policy Terms of Use 2016 Encyclopædia Britannica Inc MLA style Mantellidae Encyclopædia Britannica Encyclopædia Britannica Online Encyclopædia Britannica Inc 2016 Web 12 Feb 2016 http www britannica com animal Mantellidae APA style Mantellidae 2016 In Encyclopædia Britannica Retrieved from http www britannica com animal Mantellidae Harvard style Mantellidae 2016 Encyclopædia Britannica Online Retrieved 12 February 2016 from http www britannica com animal Mantellidae Chicago Manual of Style Encyclopædia Britannica Online s v Mantellidae accessed February 12 2016 http www britannica com animal Mantellidae While every effort has been made to follow citation style rules there may be some discrepancies Please refer to the appropriate style manual or other sources if you have any questions Update Link Click anywhere inside the article to add text or insert superscripts subscripts and special characters You can also highlight a section and use the tools in this bar to modify existing content Add links to related Britannica articles You can double click any word or highlight a word or phrase in the text below and then select an article from the search box Or simply highlight a word or phrase in the article then enter the article name or term you d like to link to in the search box below and select from the list of results Note we do not allow links to external resources in editor Please click the Web sites link for this article to add citations for external Web sites Editing Tools Tips for Editing Leave Edit Mode Submit We welcome suggested improvements to any of our articles You can make it easier for us to review and hopefully publish your contribution by keeping a few points in mind Encyclopaedia Britannica articles are written in a neutral objective tone for a general audience You may

    Original URL path: http://www.britannica.com/animal/Mantellidae (2016-02-13)
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  • evolution - The science of evolution | scientific theory | Britannica.com
    concept of natural selection The central argument of Darwin s theory of evolution starts with the existence of hereditary variation Experience with animal and plant breeding had demonstrated to Darwin that variations can be developed that are useful to man So he reasoned variations must occur in nature that are favourable or useful in some way to the organism itself in the struggle for existence Favourable variations are ones that

    Original URL path: http://www.britannica.com/science/evolution-scientific-theory/The-science-of-evolution (2016-02-13)
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  • vertebrate | animal | Britannica.com
    phylum Chordata They have backbones from which they derive their name The vertebrates are also characterized by a muscular system consisting pimarily of bilaterally paired masses and a central nervous system partly enclosed within the backbone The subphylum is one of the best known of all groups of animals Its members include the classes Agnatha Chondrichthyes and Osteichthyes all fishes Amphibia amphibians Reptilia reptiles Aves birds and Mammalia mammals General

    Original URL path: http://www.britannica.com/animal/vertebrate (2016-02-13)
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  • scrubland | ecology | Britannica.com
    both African and Australian heathlands except through human introduction For example Australian wattles have been introduced into the South African fynbos where they pose substantial conservation problems Heathers are virtually absent from Australia their place being taken by epacrids e g Lissanthe and Melichrus and myrtles e g Homoranthus and Kunzea Again bird pollination primarily by various honeyeaters family Meliphagidae and seed dispersal by ants are common The scrublands of drier hotter regions are poorer in plant species than most Mediterranean scrublands Chenopod scrublands are dominated by shrubs in the family Chenopodiaceae these bushes also are called saltbushes because they typically grow in dry saline environments and sometimes store salt in their leaves which have a characteristically salty taste The mallees grow as multistemmed shrubs frequently more than five metres tall although some can assume tree form when conditions are not stressful In Chilean scrublands members of the daisy family such as Baccharis and Haplopappus and members of the bean family such as Acacia Dalea and Prosopis are particularly well represented together with cacti and many other plants The biota of Northern Hemisphere scrublands is distinct from that of the Southern Hemisphere The wide variety of shrubs excludes proteoids while some common groups of plants in the north such as oaks Quercus do not occur in the south Other more widespread plant groups tend to be represented in northern scrublands by distant relatives of southern plants Scrublands in Mediterranean Europe are dominated by a variety of shrubs in such families as oak heather Erica and Arbutus bean Cytisus olive Olea and pistachio Pistacia Oaks and heathers Arctostaphylos are also prominent in California as are many other shrubs including Garrya Heteromeles Rhus the blue flowered Ceanothus and the common chamiso Adenostoma fasciculatum Many shrubs that dominate scrublands in all regions are thorny and their foliage which is usually not very nutritious contains pungent and unpalatable substances that discourage browsing Larger mammals are typically absent Small mammals may be common however and they play an important role in influencing regeneration of the vegetation through eating seeds see tropical forest Population and community development and structure Birds may be varied and abundant but in Northern Hemisphere scrublands few are reliant on nectar as a food source and thus they only incidentally pollinate flowers unlike the coevolved fauna of the Australian and South African scrublands Population and community development and structure Natural scrublands occur in areas in which factors such as climate or soil are sufficiently challenging to preclude the growth of trees but are adequate to allow the growth of shrubs In many areas the shrubs belong to species that are able under more favourable conditions to develop into trees but in others they always grow into shrub form regardless of the environment In scrublands with Mediterranean climate similar vegetation structure and ecological processes have evolved from different ancestral plants and animals in mutual isolation but in response to similar climatic environments The extent to which this has occurred has been investigated substantially in the scrublands of California and Chile These areas were chosen in part because natural patterns have not been obscured by long histories of intensive human agricultural impacts This research has revealed remarkable similarities in the morphology of individual plants the structure of the overall vegetation the patterns of plant physiology and distribution along aridity gradients and the diversity and adaptations of associated fauna A profile of the typical vegetation is shown in Figure 2 Effect of agriculture on the natural development of scrublands The natural development of semiarid scrublands is affected by cultivation of the land and by grazing by domestic animals a destructive form of land use that can substantially alter vegetation Even regions that have been used for farming only for relatively short periods have been rapidly and substantially altered Scrublands that are dependent on nutrient poor soil can be permanently destroyed by the addition of nutrients such as fertilizers used for agriculture or that have entered the ecosystem as pollution The amelioration of soil conditions permits invasion by plants from other vegetation types that can grow more vigorously than the original scrubland plants In most regions of Australia agriculture including the grazing of domestic animals has been practiced for less than 200 years The semiarid tropical scrublands are reasonably intact across large areas but the more southerly chenopod scrublands have been altered markedly during the past 150 years by intense sheep grazing Ninety percent of these valuable rangelands have degenerated to some extent and 25 percent have been severely affected During this process 22 plant species have become so rare that they are considered to be in danger of extinction within the next 10 to 20 years if land use practices are not changed The number of palatable species seems to be declining especially near watering points although by contrast some unpalatable shrub species have become more common This effect has greatly reduced the value of the vegetation as rangeland Large areas of mallee have been cleared to grow wheat although the climate is so marginal that the crop often fails completely Other Australian scrublands also continue to be cleared and fertilized for various agricultural purposes Broad scale farming activity may create as well as destroy scrublands This has happened in large areas of semiarid eastern Australia where grasslands were formerly dominant Previously when thick grass grew after a rain it became fuel for wildfires a process that favoured regrowth of the grass after the next rain but killed many shrub seedlings After introduction of large numbers of domestic stock and eviction of the area s Aboriginal inhabitants who had maintained the fire regime the grass was mostly eaten by sheep and fires became less frequent As a consequence very large areas were invaded by woody weeds unpalatable native shrubs that have created new areas of scrublands and greatly diminished the area s grazing value Wherever climatic conditions are marginal for tree growth vegetation is vulnerable to alteration by human activities Trees disappear as a result

    Original URL path: http://www.britannica.com/science/scrubland (2016-02-13)
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  • rainforest | Britannica.com
    see tropical forest perpetual rainforests MinuteEarth A Britannica Publishing Partner Rainforests usually occur in regions where there is a high annual rainfall of generally more than 1 800 mm 70 inches and a hot and steamy climate The trees found in these regions are evergreen Rainforests may also be found in areas of the tropics in which a dry season occurs such as the dry rainforests of northeastern Australia In

    Original URL path: http://www.britannica.com/science/rainforest (2016-02-13)
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  • animal | biology | Britannica.com
    level of organization Radiata a tissue level of organization Bilateria an organ level of organization Social levels of organization Form and function Support and movement The nervous system The senses Hormones Digestion Water vascular systems Reproduction and life cycles Ecology and habitats Competition and animal diversity Evolution of ecological roles Humans and the environment Evolution and paleontology Appearance of animals Rise of vertebrates Classification Diagnostic features Annotated classification Critical appraisal Animal fallow deer F Siedel Bruce Coleman Inc kingdom Animalia any of a group of multicellular eukaryotic organisms i e as distinct from bacteria their deoxyribonucleic acid or DNA is contained in a membrane bound nucleus They are thought to have evolved independently from the unicellular eukaryotes Animals differ from members of the two other kingdoms of multicellular eukaryotes the plants Plantae and the fungi Mycota in fundamental variations in morphology and physiology This is largely because animals have developed muscles and hence mobility a characteristic that has stimulated the further development of tissues and organ systems Animals dominate human conceptions of life on Earth not simply by their 100 of 15 949 words Images Videos About Us About Our Ads Contact Us Privacy Policy Terms of Use 2016 Encyclopædia

    Original URL path: http://www.britannica.com/topic/animal (2016-02-13)
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