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Key Terms

BiologyKey Terms

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
indentation formed during gastrulation, evident in the gastrula stage
16–32 cell stage of development of an animal embryo
body plan
morphology or constant 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 division of 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
developmental tissue fate of each embryonic cell is already determined
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
early stage of development when germ cells or “stem cells” are not yet pre-determined to develop into specific cell types
clade of protostomes that exhibit a trochophore larvae stage or a lophophore feeding structure
mass extinction
event 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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