Pertaining to the front, or head, of a bilaterally symmetrical animal.
The endoderm-lined cavity, formed during gastrulation, that develops into the digestive tract of an animal.
A segmented ecdysozoan with a hard exoskeleton and jointed appendages. Familiar examples include insects, spiders, millipedes, and crabs.
Body symmetry in which a central longitudinal plane divides the body into two equal but opposite halves
Member of a clade of animals with bilateral symmetry and three germ layers.
In a gastrula, the opening of the archenteronthat typically develops into the anus in deuterostomes and the mouth in protostomes.
A hollow ball of cells that marks the end of the cleavage stage during early embryonic development in animals.
A fluid- or air-filled space between the digestive tract and the body wall.
In animals, a set of morphological and developmental traits that are integrated into a functional whole—the living animal.
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. This burst of evolutionary change occurred about 535-525 million years ago.
A protective layer external to the plasma membrane in the cells of plants, prokaryotes, fungi, and some protists. Polysaccharides such as cellulose (in plants and some protists), chitin (in fungi), and peptidoglycan (in bacteria) are an important structural component of cell walls.
An evolutionary trend toward the concentration of sensory equipment at the anterior end of the body.
Member of the phylum Chordata, animals that at some point during their development have a notochord; a dorsal, hollow nerve cord; pharyngeal slits or clefts; and a muscular, post-anal tail.
A group of species that includes an ancestral species and all its descendants.
(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.
A body cavity lined by tissue derived only from mesoderm.
An animal that possesses a true coelom (a body cavity lined by tissue completely derived from mesoderm).
A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early.
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.
Having two germ layers.
Member of a group of animal phyla identified as a clade by molecular evidence. Many ecdysozoans are molting animals.
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.
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.
The domain that includes all eukaryotic organisms.
Member of a clade of animals with true tissues. All animals except sponges and a few other groups are eumetazoans.
An embryonic stage in animal development encompassing the formation of three layers: ectoderm, mesoderm, and endoderm.
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.
A group of organisms that share the same level of organizational complexity or share a key adaptation.
An organism that obtains organic food molecules by eating other organisms or substances derived from them.
A free-living, sexually immature form in some animal life cycles that may differ from the adult animal in morphology, nutrition, and habitat.
In some lophotrochozoan animals, including brachiopods, a crown of ciliated tentacles that surround the mouth and function in feeding.
Member of a group of animal phyla identified as a clade by molecular evidence. Lophotrochozoans include organisms that have lophophores or trochophore larvae.
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 species that have these structures.
A developmental transformation that turns an animal larva into either an adult or an adult-like stage that is not yet sexually mature.
A process in ecdysozoans in which the exoskeleton is shed at intervals, allowing growth by the production of a larger exoskeleton.
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.
An animal whose body cavity is lined by tissue derived from mesoderm and endoderm.
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.
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.
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.
Possessing three germ layers: the endoderm, mesoderm, and ectoderm. Most eumetazoans are triploblastic.