154 
THE SIMPLER METAZOA 
not the cells of the digestive tract alone. Certain other 
cells are modified in such a way for secreting and holding 
lime that they form bones by which the whole body is bene¬ 
fited. 
Some cells are grouped to form muscles to be used in 
securing food and in enabling animals to escape from their 
enemies. Other cells are for the purpose of conveying and 
Figure 144. — Bath Sponge. A Skeleton. 
interpreting impressions, so that the animal may hear the 
approach of an enemy, or detect the presence of food. It is 
largely the carrying out of this “ division of labor ” that tells 
us the rank of an animal or a plant in biological classification. 
In the business world we have a somewhat similar division 
of labor. Years ago the cobbler made all the parts of a shoe. 
In our large shoe factories to-day we find no one man making 
an entire shoe. One man runs the machine that cuts the 
leather and he does no other part of the work. He may have 
been a cutter twenty years, and he works rapidly and ac- 
