GLOSSARY 509 



Response. Any change in the activity of protoplasm, and therefore of an 

 organism as a whole, as the result of a stimulus. See Irritability. 



Resting Cell. One which is not undergoing mitosis. 



Retina. Actual percipient part of the eye by virtue of a sensory layer 

 which is stimulated by light rays. 



Reversion. The appearance of an ancestral character in an individual 

 after it has been 'latent' for one or many generations. Atavism. 



Rotifera. Microscopic, aquatic, multicellular animals. Wheel animal- 

 cules. 



Rusts. Fungi which are destructive parasites of the higher plants; e.g., 

 the Wheat Rust. 



Sacculus. The anterior sac of the labyrinth of the ear, a derivative of 



which becomes the cochlea in higher Vertebrates. 

 Saprophytic. Type of nutrition involving the absorption of complex 



products of organic decomposition; e.g., in many groups of Bacteria 



and other Fungi, as well as various species of lower animals. Saprozoic. 



See Holozoic and Autotrophic. 

 Seraceous Glands. Glands which elaborate a fatty substance (sebum) 



and secrete it into the hair follicles. Oil glands. 

 Secondary Sexual Characters. Differences between the sexes, other 



than those of the gonads and related organs. 

 Secretion. A substance elaborated by glandular epithelium; or the 



process involved. See Gland and Excretion. 

 Segregation. The distribution of homologous chromosomes, and there- 

 fore of homologous genes (allelomorphs), to separate cells during the 



formation of the gametes. The chief factor of Mendelian inheritance. 



See Reduction. 

 Semicircular Canals. Portion of the Vertebrate ear devoted to the 



maintenance of equilibrium. 

 Seminal Receptacles. Sacs within the body cavity of certain animals 



(e.g., Earthworm), which receive sperm from another individual and 



retain them until fertilization is to occur. 

 Septa. The partitions which divide the coelom of the Earthworm into a 



series of chambers, or segments. 

 Serial Homology. Homology of a structure of an organism with another 



of the same organism; e.g., appendages of the Crayfish, fore and hind 



limbs of Vertebrates. 

 Sessile. Attached, sedentary; e.g., Sponges. 

 Setae. Bristle-like structures which protrude from the body wall and 



aid in locomotion; e.g., in Earthworm and Nereis. 

 Sex-linked Characters. Characters represented by genes in the X 



chromosome. 



