Vein.
(1) A blood vessel that carries blood towards the heart. Most veins carry deoxygenated blood (the pulmonary vein is an exception). The largest veins are fed by smaller ones, which are formed by the merger of venules. Veins have thin walls and a relatively large internal diameter. Valves within the veins ensure that the flow of blood is always towards the heart. Compare artery.
(2) A vascular bundle in a leaf (see venation).
(3) Any of the tubes of chitin that strengthen an insect’s wing.

Venacavainferior (Posterior Vena-Cava, Postacaval Vein). main vein of tetrapod vertebrates passing blood into the heart from veins of almost all body behind the fore-limbs. A single median vein, the largest in the body, has no homologue in fish, whose posterior cardinal veins serve same purpose.

Vena Cava Superior (Anterior Vena-Cava, Precavel Vein). Main vein of tetrapod vetrtebrates returning blood to the heart (to right auricle) from the fore-limbs and head. Generally a right and left pair, but in many mammals, including man, only the right one persists in adult.

Vernation. Leaf arrangement in a bud.

Vesicle. Any small sac-like structure.

Vertebrada (Craniata). The largest subphylum of the Chordata, comprising all those animals with backbones (see vertebral column). The replacement of the rigid notochord by the more flexible backbone has permitted vertebrates a greater degree of movement and subsequent improvement in the sense organs and enlargement of the brain is enclosed in a skeletal case, the cranium.
There are seven extant classes; Agnatha (jawless fishes), Elasmobranchii (cartilaginous fishes), Osteichthyes (bony fishes), Amphibia, Reptilia, Aves (birds), and Mammalia.

Vessel. (Tracha). Non-living element of xylem consisting of a tube-like series of cells arranged end to end, running parallel with long axis of the organ in which it lies and in communication with adjacent element by means of numerous pits in side wall.

Vestigial Organ. Organ, the size, structure, and function of which have diminished and simplified in the course of evolution until only a trace remains. Such vestiges may still have important function in organizing embryonic development, even if their former adult function has disappeared.