| A | B |
|---|
| collagen | three-stranded protein, unique to animals, that combines to form strong, flexible fibers |
| homeotic gene | gene that controls early development in animals; contains a specific sequence of 180 nucleotides called a homeobox (also called Hox genes) |
| homeobox | a specific sequence of 180 nucleotides that is found within a homeotic gene |
| vertebrate | animal with an internal segmented backbone |
| invertebrate | animal without a backbone |
| animal phylum | group of animals defined by structural and functional characteristics that are different from every other animal phylum |
| bilateral symmetry | body plan of some organisms in which the body can be divided equally along only one plane |
| radial symmetry | arrangement of body parts in a circle around a central axis |
| protostome | animal development in which the animal's mouth develops before the anus |
| deuterostome | animal development in which the animal's anus develops before the mouth |
| sessile | unable to move from a fixed point |
| filter feeder | animal that eats by straining particles from water |
| polyp | tube-shaped body form of a cnidarian in which the mouth and tentacle face upward |
| medusa | umbrella-shaped body form of a cnidarian in which the mouth and tentacles are on the underside |
| mesoglea | jellylike material that separates the two tissue layers of a cnidarian |
| nematocyst | capsule containing a thin, coiled tubule with a poisonous barb at one end |
| gastrovascular cavity | saclike digestive space |
| complete digestive tract | digestive system that has two openings, a mouth and an anus, that are at opposite ends of a continuous tube |
| radula | file-like feeding organ found in mollusks |
| hemocoel | open space between cells in animal tissues |
| segmentation | repeated sections of an animal's body that contain the same set of body structures, apart from its distinct head and tail region |
| coelom | fluid-filled space that is completely covered by mesoderm |
| cuticle | in animals, a tough exoskeleton made of nonliving material |
| pseudocoelom | fluid-filled space with mesoderm only on one side of the space |
| ossicle | small bone |
| water vascular system | system of water-filled canals that extend down each arm of an echinoderm, such as a sea star |
