ECONOMIC IMPORTANCE 
167 
with these coral animals. The lower portion is made up of 
skeletons only. Succeeding generations build upon the work 
of their ancestors. 
Corals reproduce much as trees produce branches, but at 
certain periods eggs and sperms are produced as in the sea- 
anemone. Then the embryo settles down, secretes its own 
skeleton, and this is added to the work of other corals. 
Sea-fans and sea-plumes are coelenterates which have the 
forms suggested by their 
names. A dried speci¬ 
men of either looks as 
. if a branch had been 
dipped in a solution and 
coated. The interior is 
of a horny substance. 
The exterior is covered 
with a limy secretion. 
148. Economic Im¬ 
portance. — The corals 
alone of the coelente¬ 
rates are of economic 
importance; they add 
to many islands, protect others from being washed away, 
and in some cases form entirely new islands. 
SUMMARY 
The hydra-like animals represent an advance in the divi¬ 
sion of labor. The layers of their bodies are more definite 
and do their work better than in the sponges. Hydroids 
and the corals illustrate the formation of a colony. In some 
of the colonies the division of labor is more extensive than in 
others. The economic importance of some corals has been, 
and continues to be, very great, as they contribute to the 
formation of land in the ocean ; on the other hand, such corals 
as sea-fans serve merely as odd ornaments. 
