GLOSSARY 397 



Pylorus (pi W rus). The opening from the stomach to the intestine. 

 Pyridoxin. "N^itamin Be, the antidermatitis vitamin. 



Quadrate. One of the bones of the skull; in birds and reptiles and bony fishes, the 

 bone from wliich the lower jaw is suspended. 



Race. A group of individuals having certain characteristics in common because of 



common ancestry. 

 Radial canal. One of four tubes extending from the middle to the margin of a medusa. 

 Radial symmetry. An arrangement of the parts of an object or organism such that 



it is capable of being divided into halves that are mirrored images of one another, 



by two or more planes all of which pass through a common longitudianl axis. 

 Radiating canal. One of a series of collecting channels surrounding the pulsating 



^■acuoles of Paramecium and similar protozoa. 

 Radical. A group of atoms behaving as a unit in reactions. 

 Radio-ulna. The fused radius and ulna of frogs and toads. 

 Radius. The bone of the lower arm located on the thumb side in Ampliibia and 



the higher vertebrates. 

 Rana. A genus of frogs. R. cantabrigensis, the wood frog; R. catesbeiana, bidl 



frog; R. clamitans, green frog; R. palustris, pickerel frog; R. pipiens, leopard frog. 

 Range. The area occupied by a species or larger taxonomic group of animals or 



plants. 

 Ray, John. English naturaHst, 1627-1705. 

 Reaction. Any response of an animal to a stimulus; also any chemical change taking 



place in a substance, particularly a change involving some other substance as well. 

 Recapitulation theory. See biogenetic law. 

 Receptor. An organ which is especially sensitive to certain stimuh and serves to 



initiate impulses in nerve fibers. 

 Recessive. Not being produced when the gene for a contrasted dominant character is 



also present; said of inherited characters that are not developed in heterozygotes. 

 Reciprocal. Involving the same types of individuals, but with the sexes reversed; 



said of two crosses, in one of wliich the female possesses the same characteristic 



as does the male in the other cross. 

 Rectum. The terminal portion of the large intestine in the higher vertebrates. In 



vertebrates with a cloaca, the term is sometimes applied to the part of the large 



intestine anterior to the cloaca. 

 Reduction. Cell division in which chromosomes are not duplicated but merely 



separated from one another after having previously come together in pairs, as 



occurs in one of the two divisions in the ripening of most germ cells. 

 Reflex. Same as reflex action. 

 Reflex action. An action performed as a result of an impulse which passes over a 



reflex arc. 

 Reflex arc. A group of two or more neurons, one of them afferent, another efferent, 



so connected as to be able to transmit impulses resulting in reflex actions. 

 Regeneration. The production of lost parts by organisms. 

 Relict. A li\ing remnant of an otherwise extinct group of organisms. 

 Renal corpuscle. One of numerous bodies in the kidneys of vertebrate animals, each 



composed of the expanded end of a kidney tubule (Bowman's capsule) and an 



enclosed knot of blood capillaries (glomerulus). 

 Rennin. An enzyme i)roduced by the gastric glands and having the property of 



coagulating milk. 

 Reproduction. Tlie formation of new individuals among organisms. 



