All terms in this list:

appendage: structure such as a claw, leg, or antenna that grows from the body.

arthropod: bilaterally symmetrical animal with jointed appendages, a protective exoskeleton, and a segmented body.

closed circulatory system: a type of blood-circulation system in which blood is transported through blood vessels rather than surrounding the organs.

cnidarian: radially symmetrical, hollow-bodied animal with two cell layers organized into tissues.

exoskeleton: rigid, protective body covering of an arthropod that supports the body and reduces water loss.

gill: organ that allows a water-dwelling animal to exchange carbon dioxide for dissolved oxygen in the water.

mantle: thin layer of tissue that covers a mollusk's body and that can secrete a shell.

medusa: free-swimming, bell-shaped body form in the life cycle of a cnidarian.

metamorphosis: change of the body form that can be complete (egg, larva, pupa, adult) or incomplete (egg, nymph, adult)

mollusk: soft-bodied, bilaterally symmetrical invertebrate with a large, muscular foot, a mantle, and an open circulatory system; usually has a shell.

open circulatory system: a type of blood circulation that lacks blood vessels and in which blood washes over the organs.

polyp: vase-shaped, usually sessile body form in the life cycle of a cnidarian.

radula: scratchy, tonguelike organ in many mollusks that has rows of teethlike projections used to scrape and grate food.

symmetry: arrangement of individual body parts; can be radial (arranged around a central point), bilateral (mirror-image parts), or asymmetrical.