392 PRINCIPLES OF ANIMAL BIOLOGY 



Oocyte (o' o site). A female germ cell subsequent to the initiation of oogenesis and 

 prior to the second division. An oocyte is designated primary during the growth 

 period and prior to the first division; secondary after the first division and before 

 the second. 



Oogenesis (o' o jen' e sis). The series of changes imdergone by female germ cells 

 in preparation for reproduction. 



Oogonium (o' o go' ni urn). One of the early germ cells of a female animal, prior to 

 the beginning of oogenesis. 



Operculum (o per' ku lum). A fold of skin covering the gills and gill clefts in some 

 amphibian larvae; also a similar covering of the gills in fishes. 



Ophiuroidea {o' fi u roi' de a) . A class of Echinodermata, comprising the brittle 

 stars. For definition see Chap. 19. 



Ophthalmozoa {of thai' mo zo' a). A group of animals (literally, eye animals) in 

 Oken's early classification. The term was synonymous with Thricozoa and 

 comprised the mammals. 



Opisthocoelous (o pis tho see' lus). Having the centrum concave behind and convex 

 in front; said of vertebrae. 



Optic nerve. The nerve of sight. 



Oral groove. The spiral depression on one side of Paramecium, leading to the gullet. 



Order. A group of animals forming a subdivision of a class, and being composed 

 of one or more families. 



Ordovician. Of early Paleozoic time, succeeding the Cambrian. 



Organ. A group of cells or tissues performing some specific function. 



Organism. A living being, whether plant or animal. 



Organismal theory. The theory that parts of an organism owe their nature to the 

 nature of the whole. 



Organizer. A substance which controls some feature of embryonic development. 



Origin. The place of attachment of the proximal end of a muscle. 



Ornithology. The zoology of birds. 



Orohippus. One of the earliest known ancestors of the horse, an animal of Eocene 

 time in North America. 



Orthoceras (or thos' er as). A genus of extinct cephalopods of the orthocone type. 



Orthocone. One of the early cephalopods that lived in a straight shell. 



Osculum. An opening through which water leaves the passages of a sponge. 



Osmosis. The diffusion of a substance through a membrane in response to unequal 

 distribution of that substance on opposite sides of the membrane. 



Osmotic pressure. Objectively defined, the pressure that will just prevent diffusion 

 of a solvent into a solution when the two are separated by a semipermeable mem- 

 brane. Also, the pressure due to the greater kinetic energy of the molecules of a 

 solvent on one side of a semipermeable membrane than on the other, due to the 

 presence of a solute on the side exhibiting the lesser kinetic energy of the solvent. 



Otozoa. .\ group of animals (literally ear animals) in Oken's early classification. 

 It comprised the birds. 



Ovary. The organ in which the immature germ cells of a female animal are 

 lodged. 



Oviduct. A tube through which the eggs of a female animal leave the body. 



Oviparity. The condition of being oviparous. 



Oviparous (o vip' a rus). Egg-laying. 



Oviposition. The laying of eggs. 



Ovisac. A chamber for the storage of eggs. b(>ing in some cases a lateral pouch of 

 the oviduct, as in the earthworm. 



Ovoviviparity {o' vo viv' i par' i ti). The condition of being ovoviviparous. 



