Glossary of Biological Terms



ultimate causation

The hypothetical evolutionary explanation for the existence of a certain pattern of animal behavior.

unsaturated fatty acid

A fatty acid possessing one or more double bonds between the carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms attached to the carbon skeleton.


[Gk. ouron, urine]
A soluble form of nitrogenous waste excreted by mammals and most adult amphibians.


[Gk. from ourein, to urinate]
A duct leading from the kidney to the urinary bladder.


[Gk. from ourein, to urinate]
A tube that releases urine from the body near the vagina in females or through the penis in males; also serves in males as the exit tube for the reproductive system.

uric acid

[Gk. ouron, urine]
An insoluble precipitate of nitrogenous waste excreted by land snails, insects, birds, and some reptiles.


[Gk. ouron, urine]
The liquid waste filtered from the blood by the kidney and stored in the bladder pending elimination through the urethra.


A chordate without a backbone, commonly called a tunicate, a sessile marine animal.


[L. womb]
A female reproductive organ where eggs are fertilized and/or development of the young occurs.