GENERAL ZOOLOGY 



PARASITISM. A form of symbiosis, consisting of an association between organisms 

 of dilTerent species in which only one, the parasite, is benefited; the presence 

 of the parasite is usually detrimental to its host. CJ. COMMENSALISM, SYMBI- 

 OSIS, and MUTUALISM. 



PARENCHYMA. A loose, mesh-like tissue of irregularly shaped cells, of mesodermal 

 origin, lying between the gut wall and the body wall in acoelomate animals. 



PARIETAL. Pertaining to wall; specifically, to the lining of the coelom. 



PARTHENOGENESIS. The development of an egg without union with a spermato- 

 zoon; may be natural or follow an artificial activation. 



PEDAL. Pertaining to the foot. 



PEDOGENESIS. Reproduction by individuals that are not adult. 



PELAGIC. Living at or near the surface of the ocean at some distance from land. 



PELLICLE. A delicate layer surrounding a cell, as in some Protozoa. 



PENIS {pi. PENES). .'\n organ functioning during sexual union and characteristic 

 of the males of many species; of diverse structure and embryonic origins. 



PERI-. Combining form, meaning "around" or "near." 



PERICARDIAL. Surrounding the heart. 



PERICARDIUM. Ihe peritoneum of the pericardial cavity; also used for the peri- 

 cardial cavity and its walls. 



PERIPHERAL. Related to or situated on or toward the surface. 



PERISTOME. Membranous area surrounding the mouth. 



PERITONEUM. The compact cellular layer, usually squamous and of mesodermal 

 origin, that forms the continuous lining of the body cavity in eucoelomate 

 animals. 



PHENOMENON (/;/. PHENOMENA). A directly observed fact. 



PHYLOGENETIC. Related to the origin and evolution of a species or other group of 

 organisms. CJ. ONTOGENETIC. 



PHYLUM {pi. PHYLA). One of the major subdivisions of the Animal and Plant 

 Kingdoms. 



PHYSIOLOGICAL BALANCE. Functional unity of a cell or organism, resulting either 

 from lack of cell specialization or from well-developed coordination. 



PHYSIOLOGY. The science of function. 



PIGMENT. Organic coloring matter. 



PLACENTA {pi. PLACENTAE). The organ by means of which the mammalian embryo 

 is nourished during development; derived in part from the uterine wall and in 

 part from embryonic membranes. 



PLASMA. The liquid portion of the blood and lymph. 



PLEURAL. Pertaining to the cavity surrounding the lungs or to the membrane lining 

 such a cavity. 



POLAR AXIS. An axis between two opposite poles. CJ. POLARITY. 



POLAR BODY. A small, non-functional cell produced at each meiotic division in 

 oogenesis; there will be three of these if the first polar body divides at the time 

 of the second meiotic division. 



POLARITY. A characteristic resulting from dilTercntiation between the two ends of a 

 cell or of an organism. 



POLLINATE. To convey pollen, produced by male organs, to the female organs in 

 plants. 



POLOCYTES. I'he polar bodies. 



POLY-. Combining form, meaning "many." 



POLYEMBRYONY. The development of several embryos from one zygote. 



POLYP. Any animal with many foot-like processes; specifically, one of the individ- 

 uals of a coelenterate colony. 



POPULATION. A group of individuals of the same species that do not have the 

 same genetic constitution and consequently give rise to new combinations in 



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