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Invertebrates Animals that lack a backbone. Vertebrates Animals that possess a backbone. Spherical symmetry An organism possesses spherical symmetry if it can be cut into two identical halves by any cut that runs through the organism's center. Radial symmetry An organism possesses radial symmetry if it can be cut into two identical halves by any longitudinal cut through its center. Bilateral symmetry An organism possesses bilateral symmetry if it can only be cut into two identical halves by a single longitudinal cut along its center which divides it into right and left halves. Epidermis An outer layer of cells designed to provide protection Mesenchyme The jellylike substance that separates the epidermis from the inner cells in a sponge. Collar cells Flagellated cells that push water through a sponge. Amoebocytes Cells that move using pseudopods and preform different functions in different animals. Gemmule A cluster of cells encased in a hard, spicule-reinforced shell Polyp The sessile, tubular form of a cnidarian with a mouth and tentacles at one end and a basal disk at the other. Medusa A free-swimming cnidarian with a bell-shaped body and tentacles. Epithelium Animal tissue consisting of one or more layers of cells that have only one free surface, because the other surface adheres to a membrane or other substance. Mesoglea The jellylike substance that separates the epithelial cells in a cnidarian. Nematocysts Small capsules that contain a toxin which is injected into prey or predators. Testes Organs that produce sperm. Ovaries Organs that produce eggs. Anterior end The end of an animal that contains its head Posterior end The end of an animal that contains its tail. Circulatory system A system designed to transport food and other necessary substances throughout a creature's body. Nervous system A system of sensitive cells that respond to stimuli such as sound, touch and taste. Ganglia (singular ganglion) Masses of nerve cell bodies. Hermaphroditic Possessing both the male and female reproductive organs. Regeneration The ability to regrow a missing part of the body. Mantle A sheath of tissue that encloses the vital organs of a mollusk, makes the mollusk's shell and preforms respiration. Shell A tough, multilayered structure secreted by the mantle, generally used for protection but sometimes for body support. Visceral hump A hump that contains a mollusk's heart, digestive and excretory organs. Foot A muscular organ that is used for locomotion and takes a variety of forms depending on the animal. Radula An organ covered with teeth that mollusks use to scrape food into their mouths. Univalve An organism with a single shell. Bivalve An organism with two shells. Sponge support structure Spicules (made of lime or silica and makes sponge feel hard and spiny) or spongin (made of a tough web of protein that makes sponge feel soft)