GLOSSARY 395 



Platelet, See blood platelet. 



Plato. A Greek pliilosopher, pupil of Socrates and teacher of Aristotle. Lived 



about 427-347 b.c. 

 Platyhelminthes {plat' i hel min' theez). The phylum of flatworms. For definition 



see Chap. 19. 

 Pleistocene {plise' to seen). Belonging to the epoch following Pliocene in the Tertiary. 

 Pleodorina (pie' o do ri' no). A minute spherical organism composed of cells of two 



sizes embedded in a jellylike substance. P. californica {kaU i jor' ni ka), with 



numerous small cells; P. illinoisensis {iV li noi zen' sis), with four small cells. 

 Plethodon {pleth' o don). A genus of salamanders. 

 Pliny {plin' i). Roman naturahst (a.d. 23-79) and author of works on natural 



history. 

 Pliocene. Pertaining to the epoch of Tertiary time following Miocene. 

 Pliohippus. An extinct animal of Pliocene time, closely resemljling the horse. 

 Plumatella. A group of fresh-water bryozoa. 

 Pneumatophore {nu' ma to fore'). A capsule enclosing gas, serving to float a siphono- 



phore colony. 

 Podophrya (po dof ri a). A protozoon belonging to the class Suctoria. 

 Polar body. A small nonfunctional cell, one of the two cells produced Ijy each divi- 

 sion in oogenesis. 

 Polarity. The condition of exhibiting or possessing different properties in different 



parts; the condition of a cell in wliich the protoplasm is unlike in different parts 



of the cell. 

 Pole. A differentiated part or extremity, as of an egg, or of the spindle of a dividing 



cell. 

 Poloc3rte. The small cell produced at either of the divisions of oocytes in oogenesis. 



Same as polar body. 

 Polychaeta (poU i ke' la). A subclass of Chaetopoda (Annehda) including those 



marine j^^orms having numerous setae borne on fleshy outgrowths at the sides of 



the somites. Nereis, the sandworm, is an example. 

 Polymorphic. Having a variety of forms. 



Polymorphism. The existence of two or more kinds of individuals within a species. 

 Polyneuritis. A disease due to vitamin Bi (thiamin) deficiency. 

 Polyorchis (poZ' i or' kis). A genus of jelly fishes. 

 Polyp. One of the feeding individuals of a hydroid or coral colony or simple related 



form. 

 Polysaccharide {poV i sak' a ride). A carbohydrate whose molecule can be split into 



many molecules of simple sugar (monosaccharide). 

 Porcellio. A genus of sowbugs (Isopoda, Crustacea). 



Porifera {po rif er a). The phylum of animals comprising the sponges. For defini- 

 tion see Chap. 19. 

 Portal system. A blood vessel or group of vessels beginning and ending in capillaries. 

 Postcava. A large vein leading to the heart from behind or below. 

 Poterioceras {po te' ri os' er as). A genus of extinct cephalopods of the gomphoceran 



type. 

 F*recipitin {pre sip' i tin). A substance which produces a precipitate when two blood 



sera are mixed. 

 Precocial. Able to run about as soon as hatched; said of certain birds. 

 Precoracoid. A ventrally situated bone or cartilage of the pectoral girdle in Amphibia 



and some reptiles. 

 Primary. For application to spermatocytes, see spermatocyte. For application to 



oocytes, see oocyte. 



