916 TEXTBOOK OF ZOOLOGY 



Nucleus (nti'kleus), a typically spherical body within the cell that contains the 



chromatin. 

 Nymph (nimf), the larval stage of an insect which undergoes incomplete 



metamorphosis; also the larval stage of a few vertebrates. 



Ocellus (osel'us), a simple type of eye, as in some insects. 



Ommatidium (6m a tid'i um), one of the numerous rodlike units of the com- 

 pound eye. 

 Ontogeny (on toj'eni), the entire development and life history of an individual 



organism. 

 Oocyte (6'6 sit), the female germ cell before maturation is completed. 

 Oogenesis (6 6 jen'e sis), the maturation of the female germ cell. 

 Oogonium (6 6 go'ni lim), the female germ cell during the multiplication and 



growth stages of maturation. 

 Operculum (6 pur'kii lum), a fold of skin, bone, and scales, which covers the 



gills of fishes and certain Amphibia; also the bony structure closing the 



aperture of certain snail shells. 

 Organ (or'gan), an arrangement of two or more tissues as a part of the body 



which performs some specific function or functions. 

 Organism (or'gan iz'm), any independent living being. 

 Orthogenesis (or tho jen'e sis), the theory which holds that animals tend to 



develop along lines leading constantly in the same direction because 



they are determined by internal factors. 

 Osmosis (osmo'sis), diffusion of substances dissolved in fluid, through a semi- 

 permeable membrane. 

 Ossicle (os'ik'l), a small bony structure. 

 Ostiuna (os'tiiim), a mouthlike opening or entrance. 

 Otocyst (o'tosist), the primitive organ of hearing. 

 Ova (6'va), mature female germ cells. Sing., ovum (6'vum). 

 Ovary (6'vari), the female gonad. 

 Oviduct (6'vidukt), the duct for the passage of ova from the ovary to the 



exterior of the animal. 

 Oviparous (ovip'arus), pertaining to those animals which lay eggs that hatch 



after exclusion from the body. 

 Ovipositor (6 vi poz'i ter), an organ of female insects and others which serves 



in helping to deposit the egg. 

 Ovoviviparous (6 v6 vi vip'a rus), a condition of retention of the egg in the 



mother 's body where it is nourished by the yolk of the egg. 

 Ovulation (6 vu la'shun), the process of discharging mature eggs from the 



ovary. 

 Oxidation (6k si da'shun), a chemical combination of oxygen with another 



element. 



Paleozoology (pale 6 z6 6r6 ji), the science that treat." of the animals of the 

 past as represented by fossil remains. 



Parasite (par'asit), an organism that lives on or within and at the expense of 

 another organism. 



Parenchyma (pa reng'ki ma) . (See Mesenchj-me.) 



Parietal (pari'etal), pertaining to the walls of the coelom. 



Parthenogenesis (par the no jen'e sis), the development of an egg without 

 fertilization. 



Pathology (path6r6ji), the study of abnormal structures and abnormal func- 

 tioning of life processes. 



Pedal (ped'al), pertaining to the feet. 



Pedicellaria (ped i sela'ri a), pincherlike structures found over the surfaces of 

 sea urchins and starfishes. 



Peduncle (pedung'kl), the stemlike attachment of certain shells and barnacles 

 to other objects. 



