FERNS, FOSSILS AND FUEL 
crawl on the bottom of the sea, and nautilus shells sail 
over the surface. Some fish are already present. 
The most characteristic animal of the Cambrian and 
other early Paleozoic periods, however, was the trilobite, 
a flat crustacean that varied greatly in shape; in size it 
was from a few inches in length to two feet. A crustacean 
is an animal form whose skeleton is purely external. The 
flesh is inside a sort of armor which protects the soft 
part of the body and keeps it together. The trilobite 
was covered with this sort of hard shell and consisted of 
a head and many joints, which became more and more 
elaborately constructed before it died out at the end 
of the earlier Paleozoic era. It had horns, as well as all 
kinds of appendages on its joints, and must have given 
a strange appearance to the Cambrian, Ordovician, and 
Silurian seas. 
The trilobites became extinct by the end of the Silurian 
period. Perhaps they had too many enemies among the 
rapidly multiplying fish; perhaps they were the victims 
of a tendency in nature to which every family, genus, and 
species is exposed. That is, every living form has a begin¬ 
ning in the history of life on earth, and a time when it 
flourishes. Then it declines and disappears. In some 
types, this cycle may take only a comparatively short 
time in the geological ages; in others, it may stretch over 
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