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saclike body and a mouth surrounded by tentacles. Paleontologists have yet to
find animal fossils in rocks more than 600 million years old. However, they
have discovered the oldest known animal embryos of jellyfish preserved
within tiny fossilized eggs from the early Cambrian. The discovery indicates
that evidence for billion-year-old complex animals might be found by search-
ing for embryos in ancient rocks.
Primitive, radially symmetrical animals have just two types of cells, the
ectoderm and endoderm. In contrast, bilaterally symmetrical animals also have
a mesoderm and a distinct gut. During early cell division called cleavage in
bilateral animals, the fertilized egg forms two and then four cells, each of
which produces several small cells. Many types have two stages of develop-
ment, consisting of a stationary polyp attached to the seabed by its base with
tentacles and mouth directed upward, and a mobile umbrella-shaped medusa
or jellyfishlike phase with tentacles and mouth directed downward.
The corals possess a large variety of skeletal forms (Fig. 48), and succes-
sive generations built thick limestone reefs. Corals began constructing reefs in
the lower Paleozoic, forming chains of islands and barrier reefs along the
shorelines of the continents. The archaeocyathans resembled both corals and
sponges. However, they have no close relationship to any living group and
thus belong in their own unique phylum.These cone-shaped creatures formed
the earliest reefs, eventually becoming extinct in the Cambrian. Many corals
declined and were replaced by sponges and algae in the late Paleozoic due to
the recession of the seas in which they once thrived.
The coral polyp is a soft-bodied creature that is essentially a contractible
sac crowned by a ring of tentacles.The tentacles surround a mouthlike open-
ing and are tipped with poisonous stingers.The polyps live in individual skele-
tal cups or tubes called thecae composed of calcium carbonate. They extend
their tentacles to feed at night and withdraw into their thecae during the day
or at low tide to keep from drying in the Sun. The corals coexist symbioti-
cally (in conjunction) with zooxanthellae algae, which live within the polyp’s
body. The algae consume the coral’s waste products and produce organic
materials that nourish the polyp. Some coral species receive 60 percent of their
food from algae. Because the algae require sunlight for photosynthesis, corals
must live in warm, shallow water generally less than 100 feet deep and at tem-
peratures between 25 and 30 degrees Celsius.
The echinoderms, from Greek meaning “spiny skin,” were perhaps the
strangest animals ever preserved in the fossil record of the early Paleozoic.
Their fivefold radial symmetry with arms radiating outward from the center
of the body makes them unique among the more complex animals. They are
the only species possessing a water vascular system composed of internal canals
that operated a series of tube feet, or podia, used for locomotion, feeding, and
respiration.The great success of the echinoderms is illustrated by the fact that
73CAMBRIAN INVERTEBRATES