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chapter 32

Terms

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archenteron
the endoderm-lined cavity, formed during gastrulation, that develops into the digestive tract of an animal
blastopore
in a gastrula, the opening of the archenteron that typically develops into the anus in deuterostomes and the mouth in protostomes
diploblastic
having two germ layers
indeterminate cleavage
a type of embryonic development in deuterostomes in which each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo
coelom
a body cavity lined by tissue derived only from mesoderm
metamorphosis
a developmental transformation that turns an animal larva into either an adult or an adult-like stage that is not yet sexually mature
lophophore
in some lophotrochozoan animals, including brachipods, a crown of ciliated tentacles that surround the mouth and function in feeding
radial symmetry
symmetry in which the body is shaped like a pie or barrel (lacking a left and a right side) and can be divided into mirror-image halves by any plane through its central axis
gastrulation
in animal development, a series of cell and tissue movements in which the blastula-stage embryo folds inward, producing a three-layered embryo, the gastrula
determinate cleavage
a type of embryonic development in protosomes that rigidly casts the developmental fate of each embryonic cell very early
radial cleavage
a type of embryonic development in deuterostomes in which the planes of cell division that transform the zygote into a ball of cells are either parallel or perpendicular to the vertical axis of the embryo, thereby aligning tiers of cells one above the other
ecdysozoans
member of a group of animal phyla identified as a clade by molecular evidence; molting animals
larva
a free-living, sexually immature form in some animal life cycles that may differ from the adult animal in morphology, nutrition, and habitat
coelomates
an animal that possesses a true coelom
ediacaran biota
an early group of soft-bodied, multicellular eukaryotes known from fossils that range in age from 565 million to 545 million years old
blastula
a hollow ball of cells that marks the end of the cleavage stage during early embryonic development in animals
lopotrochozoans
member of a group of animal phyla identified as a clade by molecular evidence; organisms with lophophores or trochophore larvae
deuterostome development
in animals, a developmental mode distinguished by the development of the anus from the blastopore; often also characterized by radial cleavage and by the body cavity forming as outpockets of mesodermal tissue
protostome development
in animals, a developmental mode distinguished by the development of the mouth from the blastopore; often also characterized by spiral cleavage and by the body cavity forming when solid masses of mesoderm split
bilaterians
member of a clade of animals with bilateral symmetry and three germ layers
trochophore larva
distinctive larval stage observed in some lophotrochozoan animals, including some annelids and molluscs
body cavity
a fluid- or air-filled space between the digestive tract and the body wall
triploblastic
possessing three germ layers; the endoderm, mesoderm, and ectoderm
eumetazoans
member of a clade of animals with true tissues
ectoderm
the outermost of the three primary germ layers in animal embryos; gives rise to the outer covering and, in some phyla, the nervous system, inner ear, and lens of the eye
body plan
in animals, a set of morphological and developmental traits that are integrated into a functional whole-the living animal
dorsal
pertaining to the top of an animal with radial or bilateral symmetry
acoelomates
a solid-bodied animal lacking a cavity between the gut and outer body wall
cambrian explosion
a relatively brief time in geologic history when large, hard-bodied forms of animals with most of the major body plans known today appeared in the fossil record; occurring 535-525 million years ago
bilateral symmetry
body symmetry in which a central longitudinal plane divides the body into two equal but opposite halves
cleavage
a succession of mitotic cell divisions without cell growth between division cycles
cephalization
an evolutionary trend toward the concentration of sensory equipment at the anterior end of the body
posterior
pertaining to the rear, or tail end, of a bilaterally symmetrical animal
pseudocoelomates
an animal whose body cavity is lined by tissue derived from mesoderm and endoderm
ventral
pertaining to the underside, or bottom, of an animal with bilateral symmetry
anterior
pertaining to the front, or head, of a bilaterally symmetrical animal
spiral cleavage
a type of embryonic development in protostomes in which the planes of cell division that transform the zygote into a ball of cells are diagonal to the vertical axis of the embryo
gastrula
an embryonic stage in animal development encompassing the formation of three layers; ectoderm, mesoderm, and endoderm
endoderm
the innermost of the three primary germ layers in animal embryos; liness the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the digestive tract in species that have these structures
mesoderm
the middle primary germ layer in an animal embryo; develops into the notochord, the lining of the coelom, muscles, skeleton, gonads, kidneys, and most of the circulatory system in specie that have these structures

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