GLOSSARY 401 



Spinal nerve. One of the paired nerves arising by two roots from the spinal cord. 

 Spindle. A group of structures resembling threads, in the form of a spindle, formed 



in the cytoplasm of a cell during mitosis. 

 Spiracle. In frog tadpoles, an opening through which water passes out of the gill 



(•hanil)er on one side. In insects, one of a inunber of openings on the sides of 



the body tlu'ough wliich air is introduced to or ejected from the tracheae. 

 Spireme {spi' reem). The coiled or tangled thread formed by the chromatin of a cell 



prior to division. 

 Spirostomum {spi ros' to nmm). A genu^ of ciliated protozoa. 

 Splint. .\ bone at either side of the foot of the horse and some of its relatives, being 



the remnant of a lost toe. 

 Spongilla. A genus of fresh-water sponges. 



Spongin. The horny material of the skeleton of the bath sponges. 

 Spontaneous generation. Same as abiogenesis. 

 Sporadic. Occurring at irregular intervals, often without apparent reason; said of 



migration of animals. 

 Spore. One of a great variety of reproductive cells usually having protective cover- 

 ings. Often the term is limited to asexual reproductive cells. The word is often 



compounded with qualifying prefixes or preceded by quahfying adjectives. 

 Sporozoa. A class of protozoa, parasites usually without locomotor organs or mouth. 

 Sporulation. The formation of spores; sometimes applied to multiple division of the 



nucleus followed by fragmentation of the cj'tosome, which occurs in the spore 



formation of certain species. 

 Squamata (skwa ma' to). An order of reptiles to wliich the snakes, lizards, and 



cliameleons belong. 

 Squamosal. A bone of the posterolateral region of the skull of vertebrates. In the 



mammals it suspends the lower jaw, but not in the other vertebrates. 

 Squamous epithelium {skwa' ■rnus). Epithelium whose cells are low and flat. 

 Statoblast (skit' o blast). A gemmulelike bodj' by means of wlrich many Bryozoa 



reproduce asexually. 

 Steapsin {ste ap' sin). The fat-splitting enzyme of the pancreatic fluid. 

 Stegodon (steg' o don). A genus of extinct animals, related to the elephants, from 



the Pliocene of southern Asia. 

 Stegosaurus {steg' o saw' rus). A genus of dinosaurs bearing rows of plates set verti- 

 cally on the back, belonging to Jurassic and Comanchean time. 

 Stejneger, Leonhard {sti' ne ger). A living American herpetologist. 

 Stentor. A genus of ciliated protozoa. 

 Sternum. The breastbone; present in most vertebrates except fishes and some 



reptiles. 

 Stimulus. A change in the environment or some internal condition wliich produces a 



reaction in an organism. 

 Stomach. An enlargement in the anterior part of the digestive tract of many animals; 



certain phases of the digestion of food occur there. 

 Storeria. A genus of snakes. S. occipitomaculata {ok sip' i to mak' u la' ta); S. 



dekayi {de kay' i). 

 Stratified. Arranged in strata or layers; said of epithelia, geological deposits, etc. 

 Stratum (pL, strata). A laj^er; specifically, a layer of sedimentary rock. 

 Stratum corneum. The thin outermost layer of cells in the skin of certain animals 



(as the frog). 

 Striated muscle. Muscle composed of cylindrical, cross-banded, multinucleate cells 



(except in the heart). Skeletal muscles in vertebrates are of this kind. 

 Striation. A stripe; as the crosslines of voluntary muscle cells. 



