cleavage

(1) The process of cytokinesis in animal cells, characterized by pinching of the plasma membrane. (2) The succession of rapid cell divisions without significant growth during early embryonic development that converts the zygote to a ball of cells.

blastula

A hollow ball of cells that marks the end of the cleavage stage during early embryonic development in animals.

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.

gastrula

An embryonic stage in animal development encompassing the formation of three layers: ectoderm, mesoderm, and endoderm.

larva

A free-living, sexually immature form in some animal life cycles that may differ from the adult animal in morphology, nutrition, and habitat.

metamorphosis

A developmental transformation that turns an animal larva into either an adult or an adult-like stage that is not yet sexually mature.

Ediacaran biota

An early group of soft-bodied, multicellular eukaryotes known from fossils that range in age from 565 million to 550 million years old.

Cambrian explosion

A relatively brief time in geologic history when many present-day phyla of animals first appeared in the fossil record. This burst of evolutionary change occurred about 535–525 million years ago and saw the emergence of the first large, hard-bodied animals.

body plan

In multicellular eukaryotes, a set of morphological and developmental traits that are integrated into a functional whole—the living organism.

radial symmetry

Symmetry in which the body is shaped like a pie or barrel (lacking a left side and a right side) and can be divided into mirror-imaged halves by any plane through its central axis.

bilateral symmetry

Body symmetry in which a central longitudinal plane divides the body into two equal but opposite halves.

dorsal

Pertaining to the top of an animal with radial or bilateral symmetry.

ventral

Pertaining to the underside, or bottom, of an animal with radial or bilateral symmetry.

anterior

Pertaining to the front, or head, of a bilaterally symmetrical animal.

posterior

Pertaining to the rear, or tail end, of a bilaterally symmetrical animal.

cephalization

An evolutionary trend toward the concentration of sensory equipment at the anterior end of the body.

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.

endoderm

The innermost of the three primary germ layers in animal embryos; lines the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the digestive tract in species that have these structures.

diploblastic

Having two germ layers.

triploblastic

Possessing three germ layers: the endoderm, mesoderm, and ectoderm. Most eumetazoans are triploblastic.

mesoderm

The middle primary layer in a triploblastic animal embryo; develops into the notochord, the lining of the coelom, muscles, skeleton, gonads, kidneys, and most of the circulatory system in species that have these structures.

body cavity

A fluid- or air-filled space between the digestive tract and the body wall.

coelom

A body cavity lined by tissue derived only from mesoderm.

coelomate

An animal that possesses a true coelom (a body cavity lined by tissue completely derived from mesoderm).

pseudocoelomate

An animal whose body cavity is lined by tissue derived from mesoderm and endoderm.

acoelomate

A solid-bodied animal lacking a cavity between the gut and outer body wall.

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.

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.

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. As a result, the cells of each tier sit in the grooves between cells of adjacent tiers.

determinate cleavage

A type of embryonic development in protostomes 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.

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.

Eumetazoans

Member of a clade of animals with true tissues. All animals except sponges and a few other groups are this.

Bilaterians

Member of a clade of animals with bilateral symmetry and three germ layers.

Ecdysozoans

Member of a group of animal phyla identified as a clade by molecular evidence. Many are molting animals.

Lophotrochozoans

Member of a group of animal phyla identified as a clade by molecular evidence. They include organisms that have lophophores or trochophore larvae.

lophophore

In some lophotrochozoan animals, including brachiopods, a crown of ciliated tentacles that surround the mouth and function in feeding.

trochophore larva

Distinctive larval stage observed in some lophotrochozoan animals, including some annelids and molluscs.