GLOSSARY 175 



seta. A chitinous spine or bristle in annelids used in lo- 

 comotion, 

 sexual. Of or pertaining to sex. Sexual reproduction 



involves the two sexes and the two kinds of sex cells, 



spermatozoa and ova. 



skeleton. The bony framework which supports the verte- 

 brate body, 

 somite. A metamere; one of the serial segments of which 



an animal like the insect or annelid is composed, 

 spermatozoon. The male sex cell, 

 spicule. A small, spine-like skeletal body embedded in 



the wall of sponges. 

 spindle. The barrel-shaped structure of threads in a cell 



at the time of the mitotic division of the nucleus, 

 spiracle. One of the openings to the tracheae or air tubes 



of insects. 



spirillum. A spiral-like bacterium, 

 spore. An asexually produced body which gives rise to a 



new organism, 

 sternum. In vertebrates the breast bone; in arthropods 



the ventral portion of the exoskeleton of a somite, 

 stomach-intestine. The posterior portion of the digestive 



tube in annelids, with the functions both of stomach 



and intestine, 

 symmetry. Orderly and similar distribution of parts in an 



organism, 

 system. An assemblage of parts or organs essential to the 



performance of some particular function. 



tarsus. The jointed foot of an insect. 



telophase. The stage in mitosis in which the cell is divided, 

 and the nucleus reformed into a typical spherical form. 



