STRUCTURE AND FUNCTION 45 



Contracting or pulsating vacuoles, small, clear spots in 

 the cell, filled with water. In the living cell these disap- 

 pear at intervals and then appear again. 



Oral groove, a funnel-shaped groove in one side of some 

 one-celled animals, conducting food to the mouth. In 

 Paramecium it often shows as an oblique line when the 

 animal rolls. 



Gullet, the inner end of the oral groove. 



Cilia, numerous minute, vibrating, protoplasmic hairs on 

 the surface of many cells. 



Respiration, the passage of oxygen into the tissues of a 

 living organism and of carbon dioxide out of them. These 

 gases can pass through any thin, moist, organic membrane. 

 When such a membrane separates two fluids which differ 

 in the amount of oxygen they contain, oxygen passes to 

 the fluid containing the smaller amount.^ The same is true 

 of carbon dioxide. Respiration is believed to occur in all 

 living organisms. 



Digestion, the process of making food materials soluble, 

 so that they can pass through membranes and be used to 

 build up protoplasm. A few forms of cells are able to 

 take in solid food and digest it in their protoplasm, but 

 most cells can admit only fluid food. 



Fission, a method of reproduction used in all cells, by 

 which a cell divides itself into two, usually through the cen- 

 ter. In some one-celled animals this may be preceded by 

 conjugation, when two animals unite temporarily and ex- 

 change nuclear substance ; or in some forms two cells may 

 fuse and the resulting cell may divide. Budding is a form 

 of fission in which a small projection is formed on the 

 parent cell and then cut off, making a new individual. 



^Note. — This passage of fluids through membranes is known as 

 osmosis. 



