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27.1: Key Terms

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    animal without a body cavity
    bilateral symmetry
    type of symmetry in which there is only one plane of symmetry, so the left and right halves of an animal are mirror images
    opening into the archenteron that forms during gastrulation
    16–32 cell stage of development of an animal embryo
    body plan
    morphology or defining shape of an organism
    Cambrian explosion
    time during the Cambrian period (542–488 million years ago) when most of the animal phyla in existence today evolved
    cell divisions subdividing a fertilized egg (zygote) to form a multicellular embryo
    lined body cavity
    Cryogenian period
    geologic period (850–630 million years ago) characterized by a very cold global climate
    determinate cleavage
    cleavage pattern in which developmental fate of each blastomere is tightly defined
    blastopore develops into the anus, with the second opening developing into the mouth
    animal that develops from two germ layers
    clade of protostomes that exhibit exoskeletal molting (ecdysis)
    Ediacaran period
    geological period (630–542 million years ago) when the oldest definite multicellular organisms with tissues evolved
    mesoderm of deuterostomes develops as pouches that are pinched off from endodermal tissue, cavity contained within the pouches becomes coelom
    animal with a body cavity completely lined with mesodermal tissue
    group of animals with true differentiated tissues
    stage of animal development characterized by the formation of the digestive cavity
    germ layer
    collection of cells formed during embryogenesis that will give rise to future body tissues, more pronounced in vertebrate embryogenesis
    Hox gene
    (also, homeobox gene) master control gene that can turn on or off large numbers of other genes during embryogenesis
    indeterminate cleavage
    cleavage pattern in which individual blastomeres have the character of "stem cells," and are not yet predetermined to develop into specific cell types
    clade of protostomes that exhibit a trochophore larvae stage or a lophophore feeding structure
    mass extinction
    event or environmental condition that wipes out the majority of species within a relatively short geological time period
    group containing all animals
    formation of organs in animal embryogenesis
    group of animals without true differentiated tissues
    blastopore develops into the mouth of protostomes, with the second opening developing into the anus
    animal with a body cavity located between the mesoderm and endoderm
    radial cleavage
    cleavage axes are parallel or perpendicular to the polar axis, resulting in the alignment of cells between the two poles
    radial symmetry
    type of symmetry with multiple planes of symmetry, with body parts (rays) arranged around a central disk
    during development of protostomes, a solid mass of mesoderm splits apart and forms the hollow opening of the coelom
    spiral cleavage
    cells of one pole of the embryo are rotated or misaligned with respect to the cells of the opposite pole
    animal that develops from three germ layers

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