STRUCTURE 
155 
curately. Another man runs the machine which sews 
uppers to the soles. He, too, is a rapid and skillful worker. 
Other men have their special lines of work to do. In the 
end they produce more shoes and better shoes than this 
same number of men could, if they were all cobblers and each 
finished his product. 
132. Sponges. — Sponges are simple Metazoa. In them 
we find division of labor carried out in a more complex way 
Figure 145. —The Sponge Known as Grantia with Part of the 
Wall Removed. 
Water enters through minute pores in the side of the sponge and escapes 
through the large opening at the top. 
than in gonium and volvox. Simple sponges have a body 
in the form of a hollow cylinder. Water enters through the 
sides of the body and passes out through a hole in the top. 
A simple sponge, called Grantia , grows in salt water attached 
to docks or other objects submerged along the seashore. 
On examination, it will be observed that grantia is less 
simple than volvox. 
133. Structure. — Grantia is composed of three layers of 
cells which show division of labor. The inner layer is called 
the endoderm (en'do-derm). It consists of cells provided 
