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GLOSSARY 



ommatidmm, om atid'ium (Gr. dim. of omma, eye), part of the compound 



eye of an arthropod, 

 ontogeny, on tofeni (Gr. on, being; gennao, I produce), the development of 



an individual organism. 



OOCyte, 5'osTt (Gr. oon, an egg), a stage in the maturation of an egg. 

 OOgenesis, oojen'esis (Gr. oon, an egg; genesis, origin), the origin and 



development of the egg. 

 OOgonium, oogo'nium (Gr. oon, an egg; gonos, generation), an egg just 



before maturation takes place. 

 organ, or'gan (L. orgamtm, an instrument), a part of a organism having a 



specific function. 



orthogenesis, or thojen'e sis (Gr. orthos, straight ; genesis), a theory of evo- 

 lution (see p. 294). 

 osculum, os 1 ku lum (L. osculum, a little mouth), an opening through which 



water is expelled from a sponge. 

 Ostium, os'ti um (L. ostium, an opening), an opening in various organs of the 



body. 

 ovary, o'va ri (L. ovum, an egg), the organ of the female in which eggs 



develop. 



Oviduct, o'vidukt (L. ovum, an egg ; ductus, led), the duct of the ovary. 

 ovum, o'vum (L. ovum, an egg), an egg. 

 oxidation, ok si da' shon (Gr. oxus, sharp), the combination of a substance 



with oxygen. 



pa'le ontoPoji (Gr. palaios, ancient ; onto, beings; logos, dis- 

 course), the science of fossil organisms. 

 parasite, par 1 a sit (Gr. parasitos, one who eats at another's expense), an 



animal that lives in, on, or at the expense of another animal, 

 parietal, pa ri'e tal (L.. paries, wall), pertaining to the walls. 

 parthenogenesis, par' the no jen'e sis (Gr. parthenos, a virgin ; gennao, I 



produce), reproduction by means of unfertilized eggs. 

 pellicle, pel'ikl (L. pelliciila, a small skin), a thin outer covering of skin or 



cuticle, 

 psricardium, perikdr'dium (Gr. peri, round about ; kardia, the heart), a 



membranous sac surrounding the heart. 

 peristaltic, peristaFtik (Gr. peristaltikos, drawing together all round), 



applied to the waves of contraction running down the alimentary canal. 

 peristome, per'istom (Gr. peri, round about; stoma, a mouth), the part 



which surrounds the mouth or oral opening. 

 peritoneum, per'itone'um (Gr. peritonaion, what is stretched round or 



over), the membrane lining the body cavity and covering most of the 



organs in the abdomen. 



