454 ELEMENTS OF BIOLOGY 



liver, pancreas, small intestine, large intestine, and rectum, are the 



chief organs that compose the human digestive system. 

 Organelle. A diminutive of organ; applied particularly to the organs 



of Protozoa and to cell organs. 

 Organic evolution. The doctrine which holds that present-day plants 



and animals have evolved from earlier forms by descent with 



change. 

 Orthogenesis. The doctrine which holds that organic evolution is 



emergent, in that it has followed a definitely directed path that 



proceeds always from the simple toward the complex. 

 OscuLUM. An aperture in the body of Porifera out of which the water 



passes. 

 Osmosis. The passage of molecules or ions through a membrane. 

 Osmotic pressure. The pressure developed by a solution against a 



membrane impermeable to the molecules of the solute. 

 Otolith. Dense concretions in the ampullae of the semicircular canals; 



when dislocated by gravity they stimulate the nerve endings which 



record the sense of balance. 

 Ovary. The organ that produces ova. 

 Oviparous (Adjective). Egg-laying forms. 

 Ovum. A cell, capable of developing into an adult, produced by a 



female or by female gonad. 

 Parasite. An organism which inhabits and obtains its subsistence directly 



from the tissues of another living organism. Parasites may be 



external or internal, and occasional or obligatory. 

 Parthenogenesis. The development of an egg without having been 



fertilized by a spermatozoon. 

 Pasteurize. To sterilize partially by heating to a temperature just above 



the maximum that bacteria can endure. First developed by Pasteur. 

 Pathogenic (Adjective). Refers to a parasite that causes disease in its 



host. 

 Pepsin. A proteolytic enzyme occurring in the gastric juice. 

 Peptone. A second intermediate product in the digestion of proteins; 



formed from proteoses. 

 Pericardial cavity. A cavity, usually derived from the coelom, which 



contains the heart. 

 Perilymph. The liquid surrounding the membranous parts of the inner 



ear. 

 Periosteum. A layer of connective tissue that closely invests the bones. 



I 



