- acoelomate
- 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
- blastopore
- opening into the archenteron that forms during gastrulation
- blastula
- 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
- cleavage
- cell divisions subdividing a fertilized egg (zygote) to form a multicellular embryo
- coelom
- 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
- deuterostome
- blastopore develops into the anus, with the second opening developing into the mouth
- diploblast
- animal that develops from two germ layers
- Ecdysozoa
- 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
- enterocoely
- mesoderm of deuterostomes develops as pouches that are pinched off from endodermal tissue, cavity contained within the pouches becomes coelom
- eucoelomate
- animal with a body cavity completely lined with mesodermal tissue
- Eumetazoa
- group of animals with true differentiated tissues
- gastrula
- 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
- Lophotrochozoa
- 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
- Metazoa
- group containing all animals
- organogenesis
- formation of organs in animal embryogenesis
- Parazoa
- group of animals without true differentiated tissues
- protostome
- blastopore develops into the mouth of protostomes, with the second opening developing into the anus
- pseudocoelomate
- 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
- sc