shells of remarkably intricate designs, including a needlelike, rounded, or
open-network structure of delicate beauty (Fig. 20).
The ability to move about under their own power is what essentially
separates animals from plants, although some animals perform this function
only in the larval stage and become sedentary or fixed to the seabed as adults.
Mobility enabled animals to feed on plants and other animals, thus establish-
ing new predator-prey relationships. Some organisms moved about by a
thrashing tail called a flagellum, resembling a filamentous bacterium that
joined the host cell for mutual benefit. Other cells had tiny hairlike
appendages called cilia that propelled the organisms around by rhythmically
beating the water. Many, such as the ameoba, traveled by extending fingerlike
protrusions outward from the main body and flowing into them.
The earliest organisms were sulfur-metabolizing bacteria similar to those
living symbiotically in the tissues of tube worms (Fig. 21).These live near sul-
furous hydrothermal vents such as those on the East Pacific Rise and the
Gorda Ridge off the northwest Atlantic coast of the United States. Sulfur
would have been abundant on the early, hot planet, spewing from a profusion
of volcanoes mostly lying on the bottom of the ocean.
Figure 20Late Jurassic
radiolarians, Chulitna
District, Alaska.
(Photo by D. L. Jones,
courtesy USGS)ARCHEAN ALGAE