GLOSSARY 393 



Ootid (o' tid). A mature egg; that is, an egg as soon as the last maturation division 

 is complete. The term is seldom used, except in comparison with the spermatid 

 in the male. 



Ootype (o' o tipe). An organ in which, in certain flatworms, the ova are surrounded 

 by nutritive material and enclosed in a shell. 



Opalina ranarum (o' pa W na ra na' rum). A species of parasitic infusorian found 

 in the intestine of the frog. 



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 Chapter XII. 



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 com- 

 prised the mammals. 



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



in front ; said of vertebrae. 

 Optic nerve {op' tik new') . The nerve of sight. 



Order {or' der). A group of animals forming a subdivision of a class, and being 

 composed of one or more families. 



Ordovician {or' do vish' an). Of early Paleozoic time, succeeding the Cambrian. 



Organ {or' gan). A group of cells or tissues performing some specific function. 



Organelle {or' gan eV). A minute organ, a term often apphed to the organs of Proto- 

 zoa. 



Organism {or' gan iz'm). A living being, whether plant or animal. 



Organogeny {or' ga noj' e ni). The embryonic origin of organs. 



Organology {or' gan oV o ji). A study or description of organs. 



Ornithology (or' ni thoV oji). The zoology of birds. 



Orohippus {o' ro hip' pus). 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 {or' tho kone). One of the early cephalopods that lived in a straight shell. 



Orthogenesis {or' thojen' e sis). The repeated change of species in the same direction 



due to internal factors, and resulting in evolution in a "straight line." 

 Osmosis {oz mo' sis). The passage of fluids through membranes, due to their tendency 



to mix. 



Osmotic pressure {ozmot'ik). A pressure exerted in a solution by the dissolved 

 substance. 



Otozoa {o' tozo' a). A group of animals (literally, the ear animals) in Oken's early 

 classification. It comprised the birds. 



Ovary {o' va ri). The organ in which the immature germ cells of a female animal are 

 lodged. 



Over-growth {o' ver groth). The growth of a layer of cells over underlying cells. 



Ovicell {o' vi sel). An individual of Bugula which serves as a receptacle for the fertil- 

 ized egg. 



Oviduct {o'vi dukt). A tube through which the eggs of a female animal leave the 

 ovary. 



Oviparity {o' vi par' i ti). The condition of being oviparous. 



