Chapt 27 VOCAB

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

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

schizocoely

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

triploblast

animal that develops from three germ layers