Glossary 681 



R 



Radial symmetry — symmetry of an animal that is cylindrical in shape with 



the parts arranged along a longitudinal axis. 

 Receptor — a sense organ. 

 Recessive — in genetics, that member of a pair of factors, which, when both are 



present, is masked by the other. 

 Reflex — an involuntary act. 

 Retina — sensitive layer of the eye. 

 Reticulum — a network. 

 Rh factor — an antigen found in some red blood cells. 



Sagittal plane — a plane dividing the body into right and left halves. 



Saliva — the digestive secretion of the salivary gland. 



Saprozoic — feeding on food that is partially or fully dissolved. 



Sarcoplasm — the substance between the fibrillae of muscle fibers. 



Scrotum — in most mammals, the external pouch that contains the testes. 



Sebaceous glands — glands of the skin that secrete sebum, a fatty material. 



Sebum — the fatty secretion of the sebaceous glands. 



Secretion — product of a gland. 



Sella turcica — a small cavity at the base of the skull in which the pituitary gland 

 lies. 



Seminiferous — producing sperm. 



Sensory — pertaining to the sense organs ; carrying nerve impulses from the sense 

 organs to the central nervous system. 



Sere — in ecology, all the developmental stages leading to and including the climax 

 of a successional series. 



Serous membrane — a type of thin membrane such as the peritoneum and peri- 

 cardium. 



Sesamoid bone — a bone developed where friction is excessive, usually in ten- 

 dons or muscles. 



Sinus — a cavity. 



Skeleton — the supporting framework of an animal. 



Skull — the skeleton of the head of a vertebrate that protects the brain and 

 some sense organs and supports the jaws. 



Species — a division of a genus in the classification of animals, comprising a group 

 of individuals which ordinarily interbreed and have a number of characters 

 in common. 



Sperm — the male gamete. 



Spermatogenesis — the process by which the sperm is formed. 



Sphincter — a muscle surrounding a circular opening. 



Splanchnic — pertaining to the viscera. 



Spore — resistant reproductive body of some protozoans. 



Stomach — a digestive organ. 



Sternum — the breast bone. 



Stimulus— any environmental change that is able to influence the activity of 

 organisms. 



