406 PRINCIPLES OF ANIMAL BIOLOGY 



Vertebrate, adj. Possessing a backbone, n. An animal having a backbone. 



Vesalius {ve sa' li us). Belgian anatomist and court physician, 1514-1564. 



Villus {pi., villi). One of the fingerlike projections from the inner surface of the small 



intestine. 

 Virchow, Rudolf {veer' no). German physiologist and pathologist, 1821-1902. 

 Visceral. Pertaining to the viscera, or organs contained in some large cavity of the 



body ; applied in the vertebrates chiefly to the organs of the abdomen, in clams to 



the digestive organs and glands above the foot. 

 Viscosity. The resistance offered by a substance to the relative movement of its 



molecules. 

 Visual purple. A light-sensitive pigment in the retina. 

 Vitamin. One of several substances common in leafy vegetables, animal fats, and 



elsewhere, which are necessary for specific aspects of metabohsm in animals. 

 Viviparity {viv' i par' i ti). The condition of being viviparous. 

 Viviparous {vi vip' a rus). Producing young from eggs that are retained in the uterus 



of the mother, with the aid of nutrition derived from the mother through a 



placenta and umbilical cord. 

 Volvox. A small spherical organism composed of flagellated green cells embedded in 



jelly, in a single layer around a liquid interior. Sometimes regarded as an animal, 



though more properly included among plants. 

 Vorticella. A cihated protozoon attached to a contractile stalk. 



Wallace, Alfred Russel. EngUsh naturalist, 1823-1913. 



X body. An object in the cytosome of some of the early cleavage cells of Sagitta, 



which marks the germ cells. 

 X chromosome. A chromosome closely associated with the determination of sex. In 



many animals the female has two of them, the male only one. 

 Xenophanes (ze nof a neez). Greek philosopher who lived about 570-480 b.c. 

 Xerophthalmia {ze' rof thai' mi a). A dry, lusterless condition of the eyeball. 



Y chromosome. A chromosome possessed only by the males of many species. It 

 behaves in spermatogenesis much as if it were homologous with the X 

 chromosome. 



Yolk plug. The remnant of the vegetative cells last to be drawn into the interior of a 

 gastrula in certain embryos. 



Zoogeography. The branch of zoology treating of the geographical distribution of 



animals. 

 Zooid. One of the members of a hydroid or siphonophore colony. Often, in a 



restricted sense, a particular kind of individual, as a hydranth. 

 Zoology. The science of animals. 

 Zygapophysis {zi' ga pof i sis). One of four short projections, two in front and two 



behind, extending from the upper portion of a vertebra. Those of the posterior 



pair articulate with the anterior pair of the vertebra next behind. 

 Zygote, A cell or individual produced by the fusion of two cells or their nuclei in 



the process of sexual reproduction. 



