GLOSSARY 397 



Plastid (plas' tid). One of several kinds of protoplasmic bodies in cells, like the green 



bodies in plant cells, which are centers of chemical activity. 

 Plastron {plas' tron). The flat plate of bones on the ventral side of a turtle. 

 Plato {pla' to). A Greek philosopher, pupil of Socrates and teacher of Aristotle. 



Lived about 427-347 B.C. 

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



see Chapter XII. 



Pleistocene (plise' to seen). Belonging to the epoch following the Tertiary. 



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

 sizes embedded in a jelly-like substance. P. californica {kaV i for' ni ka) , with 

 numerous small cells; P. illinoisensis {W li noizen' sis), with four small cells. 



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



Pliny iplin' i). Roman naturalist (23-79 a.d.) and author of works on natural 



history. 

 Pliocene {pW a seen). Pertaining to the most recent epoch of Tertiary time. 

 Pliohippus {pW o hip' pus). An extinct animal of Pliocene time, closely resembling 



the horse. 

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



phore colony. 



Poebrotherium {po' e bra the' ri um). A genus of extinct animals resembling camels, 



recovered from the lower Oligocene. 

 Polar body {po' lar hod' i). A small non-functional cell, one of the two cells produced 



by each division in the maturation of an egg. 

 Polarity {po lar' i ti) . The condition of exhibiting or possessing different properties in 



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



parts of the cell. 

 Pole {pole). A differentiated part or extremity, as of an egg, or of the spindle of a 



dividing cell. 

 Polistes {po lis' teez). A genus of wasps. 

 Polychaeta (poV i ke' ta). A subclass of Chsetopoda (Annelida) including those 



marine worms having numerous seta? borne on fleshy outgrowths at the sides of the 



somites. Nereis, the sand-worm, is an example. 

 Polygyra {poV i ji' ra). A genus of land snails. 

 Polymorphic {poV i mar' fik). Having a variety of forms. 

 Polymorphism {poV i mar' fiz'm). The existence of two or more kinds of individual 



within a species. 

 Polyorchis {pol' i or' kis). A genus of jellyfishes. 

 Polyp {poV ip). One of the feeding individuals of a hydroid or coral colony or simple 



related form. 

 Polypeptide {pol' i pep' tid). A compound formed by the union of two or more 



molecules of amino-acids with the loss of water. 

 Polystomella {pol' i sto meV la). A genus of rhizopod Protozoa possessing a perforated 



shell. 

 Porcellio {por seV li o). A genus of sow-bugs (Isopoda, Crustacea). 

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

 tion see Chapter XII. 

 Porocyte {po' ro site). A contractile cell in sponges, whose function is the contraction 



and expansion of the water pores. 



