Undulate. With up-and-down wavy edges.

Unilocular. Made up of one cavity.

Unisexual.
(1) (Of flowering plants or flowers) having stamens and carpels in separate flowers. Can be either monoecious or dioecious.
(2) (Of an individual animal) producing either male or female gametes, but not both.

Univariate analysis. An analysis of a single character.

Urceolate. Um-shaped.

Urea. Main excreted product of amino acid break – down in ureotelic vertebrates. Occur also in plants. A nitrogen-containing organic compound, CO(NH₂)_2’ and readily soluble in water.

Ureide. Any compound formed between urea and organic acids.

Uricotelic. Excreting uric acid as main break-down product of amino acids. Characteristic of terrestrial animal which develop within a shell, unable to get rid of their excretory products, which they store in this insoluble form.

Urinary Bladder. Sac storing urine; an expansion of the kidney duct in, e.g. Crustacea, teleosts. In tetrapods, a ventral diverticulum of hind end of gut which in embryo of amniotes leads into allantois but above in adult birds and most reptiles.

Urinifrous Tubule. Narrow coiled tube of vertebrate kidney leading from Bowman’s Capsule to the collecting ducts which convey the urine to the ureter or to the Wolffian duct. The fluid filtered into the Bowman’s capsule is altered in composition as it travels along the tubule, valuable substances in it such as glucose, much of the salts, and in land vertebrates much of the water, being absorbed by the tubule walls.
Some other substances are excreted into it. Blood supply to tubule is from renal portal system in anamniotes; from renal artery via glomerulus in amniotes.