GLOSSARY 455 



Peristalsis. The rhythmic contractions of plain muscle; occurs as the 



motions of the digestive canal in Man. 

 Peritoneal cavity. That portion of the coelom which contains the 



viscera. 

 Permeability. The property of a layer or partition that determines its 



penetrability. 

 Pessulus. The vibratory member in the voice-producing apparatus of 



the bird. 

 Phagocyte. A white blood cell capable of ingesting foreign particles 



and bacteria. 

 Phagocytosis. The process of engulfing foreign particles or bacteria by 



certain white blood cells. 

 Phenotype. The external appearance of an individual with respect to 



one or more inheritable characters. 

 Photogenin. a substance which is the source of luminescence in lumi- 

 nescent animals. 

 Photophelein. a substance that reacts with photogenin in producing 



luminescence in animals. 

 Photosynthesis. A metabolic process occurring in green plants whereby 



carbohydrates are constructed by the catalytic action of chlorophyll 



on water and carbon dioxide in the presence of sunlight. 

 Phototropism. Response to light. 

 Phylogenetic (Adjective). The history of an organism as traceable 



through lower forms. 

 Physiology. The science of function. 

 Pistil. The central shaft in a flower, usually made up of more than 



one tube fused together and ending outward in a more or less 



flattened stigma. 

 Placenta. A structure composed of foetal membranes and a region of 



the wall of the uterus of mammals. It is the organ of nutrition, the 



means of excretion, and the organ for interchange of oxygen and 



carbon dioxide during the uterine life of the young of mammals. 

 Planula. a flattened plate of cells, an early stage in the development 



of members of the phylum Ccelenterata. 

 Plasma. The liquid portion of the blood. 

 Plasma membrane. Living protoplasm specialized to form the bound 



ary of the cell. 

 Plastid. a dense mass of protoplasm occurring in the cell cytoplasm. 



There may be several sorts of plastids in the same cell. 



