Historical Geology Understanding Our Planet\'s Past

ilar to the spicules of some sponges today, indicating little change had taken

place over the intervening period unlike in most other groups.The glass-barbed

sponges,however, are quite different from the types used in the bathtub.

The coelenterates, from Greek meaning “gut,” were well represented in

the Cambrian seas.They are the most primitive of animals and include jelly-

fish, sea anemones, sea pens, and corals. Most coelenterates are radially sym-

metrical, with body parts radiating outward from a central point.They have a

Figure 47Trilobite fossils
of the Cambrian age
Carrara Formation in the
southern Great Basin of
California and Nevada.
(Photo by A. R. Palmer,
courtesy USGS)

Historical Geology