318 ANIMAL LIFE 



Sperm cell : 20. 



Spiracle : one of the breathing openings of an insect situated on the 



side of the abdomen or thorax. 

 Spon'gin : 20. 

 Stimulus (pi. stimuli) : that which excites action in plant or animal 



tissue. 



Strata : layers, usually said of rocks. 

 Stridula tion : 122. 

 Sub-species : 151. 

 Symbiosis: 90. 



Tactile : referring to the sense of touch. 118. 



Tadpole: 58. 



Ten'tacle : a protruding flexible process or appendage, usually of the 



head of invertebrate animals, being used as an organ of touch, 



prehension, or motion. 

 Termitoph ilous : said of insects inhabiting the nests of termites. 



Vac'uole : a minute cavity containing air, water, or a chemical secre-. 



tion of the protoplasm, found in an organ, tissue, or cell. 

 Vestigial : 82. 

 Vis'cera : the organs in the great cavities of the body, commonly used 



for the organs in the abdominal cavity. 



Yolk (yok) : 48. 



Zoea: 59. 



ZSogebg'raphy : 147. 



Zo'oid : one of the more or less independent members of a colonial or 



compound organism. 

 Zoologist : a student of animals. 



