front 1 cleavage | back 1 (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. |
front 2 blastula | back 2 A hollow ball of cells that marks the end of the cleavage stage during early embryonic development in animals. |
front 3 gastrulation | back 3 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. |
front 4 gastrula | back 4 An embryonic stage in animal development encompassing the formation of three layers: ectoderm, mesoderm, and endoderm. |
front 5 larva | back 5 A free-living, sexually immature form in some animal life cycles that may differ from the adult animal in morphology, nutrition, and habitat. |
front 6 metamorphosis | back 6 A developmental transformation that turns an animal larva into either an adult or an adult-like stage that is not yet sexually mature. |
front 7 Ediacaran biota | back 7 An early group of soft-bodied, multicellular eukaryotes known from fossils that range in age from 565 million to 550 million years old. |
front 8 Cambrian explosion | back 8 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. |
front 9 body plan | back 9 In multicellular eukaryotes, a set of morphological and developmental traits that are integrated into a functional whole—the living organism. |
front 10 radial symmetry | back 10 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. |
front 11 bilateral symmetry | back 11 Body symmetry in which a central longitudinal plane divides the body into two equal but opposite halves. |
front 12 dorsal | back 12 Pertaining to the top of an animal with radial or bilateral symmetry. |
front 13 ventral | back 13 Pertaining to the underside, or bottom, of an animal with radial or bilateral symmetry. |
front 14 anterior | back 14 Pertaining to the front, or head, of a bilaterally symmetrical animal. |
front 15 posterior | back 15 Pertaining to the rear, or tail end, of a bilaterally symmetrical animal. |
front 16 cephalization | back 16 An evolutionary trend toward the concentration of sensory equipment at the anterior end of the body. |
front 17 ectoderm | back 17 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. |
front 18 endoderm | back 18 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. |
front 19 diploblastic | back 19 Having two germ layers. |
front 20 triploblastic | back 20 Possessing three germ layers: the endoderm, mesoderm, and ectoderm. Most eumetazoans are triploblastic. |
front 21 mesoderm | back 21 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. |
front 22 body cavity | back 22 A fluid- or air-filled space between the digestive tract and the body wall. |
front 23 coelom | back 23 A body cavity lined by tissue derived only from mesoderm. |
front 24 coelomate | back 24 An animal that possesses a true coelom (a body cavity lined by tissue completely derived from mesoderm). |
front 25 pseudocoelomate | back 25 An animal whose body cavity is lined by tissue derived from mesoderm and endoderm. |
front 26 acoelomate | back 26 A solid-bodied animal lacking a cavity between the gut and outer body wall. |
front 27 protostome development | back 27 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. |
front 28 deuterostome development | back 28 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. |
front 29 spiral cleavage | back 29 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. |
front 30 determinate cleavage | back 30 A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early. |
front 31 radial cleavage | back 31 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. |
front 32 archenteron | back 32 The endoderm-lined cavity, formed during gastrulation, that develops into the digestive tract of an animal. |
front 33 blastopore | back 33 In a gastrula, the opening of the archenteron that typically develops into the anus in deuterostomes and the mouth in protostomes. |
front 34 Eumetazoans | back 34 Member of a clade of animals with true tissues. All animals except sponges and a few other groups are this. |
front 35 Bilaterians | back 35 Member of a clade of animals with bilateral symmetry and three germ layers. |
front 36 Ecdysozoans | back 36 Member of a group of animal phyla identified as a clade by molecular evidence. Many are molting animals. |
front 37 Lophotrochozoans | back 37 Member of a group of animal phyla identified as a clade by molecular evidence. They include organisms that have lophophores or trochophore larvae. |
front 38 lophophore | back 38 In some lophotrochozoan animals, including brachiopods, a crown of ciliated tentacles that surround the mouth and function in feeding. |
front 39 trochophore larva | back 39 Distinctive larval stage observed in some lophotrochozoan animals, including some annelids and molluscs. |