chains of islands and altering the shorelines of the continents. Reef-building
corals created the foundations for spectacular underwater edifices that today
cover about 750,000 square miles and house about one-quarter of all marine
species.The corals diverged into two basic lineages before they developed cal-
cified skeletons, suggesting they might have evolved a reef building ability
twice in geologic history. Many corals declined in the late Paleozoic and were
replaced by sponges and algae when the seas they inhabited receded.
The bryozoans (Fig. 64), often called moss animals, resemble corals on a
smaller scale. However, they are more closely related to brachiopods. They
consist of microscopic individuals living in small colonies up to several inches
across, giving the ocean floor a mosslike appearance. Bryozoans are retractable
creatures, encased in calcareous vaselike structures into which they retreat for
safety when threatened. Living species occupy seas at various depths, with cer-
tain rare members adapted to life in freshwater.
A single free-moving larval bryozoan establishes a new colony by fixing
onto a solid object and growing into many individuals by a process of bud-
ding,which is the production of outgrowths.The polyp has a circle of ciliated
tentacles, forming a sort of net around the mouth and used for filtering micro-
scopic food floating by. The tentacles rhythmically beat back and forth, pro-
ducing water currents that aid in capturing food that is digested in a U-shaped
gut.Wastes are expelled outside the tentacles just below the mouth.
Fossil bryozoans are common in Paleozoic formations, especially those
of the American Midwest and Rocky Mountains. Bryozoan species are iden-
Figure 63A collection of
coral at Saipan,Marshall
Islands.
(Photo by P. E. Cloud,
courtesy USGS)ORDOVICIAN VERTEBRATES