A hard structure on the outside of an organism's body. This does not grow with the organism and must be shed periodically.

This is also called a hydrostatic skeleton in which an animal uses its watery environment to fill a central cavity inside its body to build up pressure.

An arrangement of physical features that contribute to structure and form.

This type of symmetry divides an organism into mirror image halves (left and right sides). Only one line is possible to divide it.

This type of symmetry divides an organism from top to bottom and can have multiple lines of symmetry.

An organism or a life phase which does not move freely ; it is attached to something else (like the ocean bottom or a rock).

An animal that has a backbone or vertebral column.

From Phylum Chordata with a notochord or nerve cord (spinal cord) in some part of the life cycle.

An animal without a backbone or vertebral column.

Can regulate its body temperature from the inside (stays relatively the same temperature in any environment). Warm blooded.

The body temperature of these type of organisms is close to the temperature of the environment. Cold blooded.

After fertilization occurs, the zygote undergoes division and forms a hollow ball of cells called this.

The cells in the wall of the blastula begin to move into the inside of the ball through a hole called the blastopore to form layers of cells called these in the animal embryo (often three of these: endoderm, mesoderm, and ectoderm).

The innermost germ layer that produces the digestive tract in an animal.

The middle germ layer that forms the muscle and circulatory systems among other things in an animal.

The outermost germ layer that forms the animal's outer covering and related systems.

The concentration of sense organs and nerve cells at the front of an animal's body (something like a head).

This allows the animal to expel wastes and is typically at the opposite end of the body from the "head" area.

When a sperm and egg cell unite to form a zygote outside of the mother's body (spawning in fish is an example of this).

When a sperm and egg cell unite to form a zygote inside the mother's body (embryo may develop inside mother's uterus or be packaged into an egg with a shell).

When eggs containing embryos are hatched outside of body of the parent (birds, most fish, amphibians, insects).

When eggs containing embryos hatch inside parent's body or the embryo develops inside the mother but is not directly connected to her (some fish and amphibians).

When young are born alive after developing inside mother's body and drawing nutrients from her (most mammals, some fish and amphibians, and a few insects).

The way an animal responds to its environment and to stimuli.

An unlearned, often highly complex behavior such as a mating ritual or seasonal migration (innate).