462 THE ORIGIN OF SPECIES 



be drawn across the eye in Birds and Reptiles, either to mod- 

 erate the effects of a strong light or to sweep particles of dust, etc., 

 from the surface of the eye. 

 Neuters. — Imperfectly developed females of certain social insects 

 (such as Ants and Bees), which perform all the labors of the com- 

 munity. Hence they are also called workers. 



Ocelli. — ^The simple eyes or stemmata of Insects, usually situated on 



the crown of the head between the great compound eyes. 

 CEsoPHAGUS. — The gullet. 

 Oolitic. — ^A great series of secondary rocks, so called from the texture 



of some of its members, which appear to be made up of a mass of 



small egg-like calcareous bodies. 

 Operculum. — A calcareous plate employed by many MoUusca to close 



the aperture of their shell. The opercular valves of Cirripedes are 



those which close the aperture of the shell. 

 Orbit. — The bony cavity for the reception of the eye. 

 Organism. — ^An organized being, whether plant or animal. 

 Orthospermous. — ^A term applied to those fruits of the Umbelliferae 



which have the seed straight. 

 Osculant. — ^Forms or groups apparently intermediate between and 



connecting other groups are said to be osculant. 

 Ova. — ^Eggs. 

 Ovarium or Ovary (in plants). — ^The lower part of the pistil or female 



organ of the flower, containing the ovules or incipient seeds; by 



growth after the other organs of the flower have fallen, it usually 



becomes converted into the fruit. 

 OviGEROUS . — Egg-bearing. 

 Ovules (of plants). — ^The seeds in the earliest condition. 



Pachyderms. — ^A group of Mammalia so called from their thick skins, 



and including the Elephant, Rhinoceros, Hippopotamus, etc. 

 Palaeozoic. — The oldest system of fossiliferous rocks. 

 Palpi. — ^Jointed appendages to some of the organs of the mouth in 



Insects and Crustacea. 

 pAPiLiONACEiE. — An Order of Plants (see LEGUMiNOSiE). — ^The flowers 



of these plants are called papilio'naceous, or butterfly-like, from the 



fancied resemblance of the expanded superior petals to the wings 



of a butterfly. 

 Parasite. — ^An animal or plant living upon or in, and at the expense of, 



another organism. 

 Parthenogenesis.— The production of living organisms from unim- 



pregnated eggs or seeds. 



