542 GENERAL BIOLOGY —A REVIEW AND SUMMARY 
g. Excretion. — The energy used by organisms in their 
several activities results in the formation of various wastes. 
These wastes are usually in the form of a gas, as carbon 
dioxide, given off in respiration by animals and plants, or as 
a liquid when discharged from the sweat glands and kidneys. 
Review the following : page 5; sections 8, 47, 61, 73, 116 ? 
122, 142, 165, 176, 237, 249, 358; and figure 73. 
h. Sensation. — We do not know whether we have more 
than five senses, but no matter how many there may be, all 
require that special cells respond quickly to various stimuli, 
such as light, heat, sound, etc. Such cells help man to 
understand his environment, and the more he trains his 
sense organs, the greater will be his capacity to enjoy nature. 
If our sense organ cells were not connected with the brain by 
nerves, they would be of but little use to us. Man has the 
most highly specialized brain of any animal. 
Most animals have sense organs and some form of a 
nervous system. The presence of such organs indicates a 
form of division of labor. But there are animals without 
sense organs or a definite nervous system. Such animals 
rely upon the general sensitiveness of the protoplasm to 
make them aware of changes in their environment. Plants 
do not have sense organs or a nervous system, yet they 
respond to light and dark, summer and winter. 
Review the following: page 6; sections 6, 49, 62, 74, 77, 
110, 123, 143, 177, 363; and figure 74. 
i. Reproduction. — The unicellular plants and animals 
reproduce in a very simple manner by fission. In multi¬ 
cellular organisms reproduction begins in a single cell, the 
germ cell. This germ cell has the power to carry on a pro¬ 
gram of development that varies but little for all eggs of one 
kind. The fish egg and the frog egg look much alike, but 
each follows its own individual order in development until 
the adult stage is reached. There is no mistake, and the 
development is never interrupted, and there can be no pause 
