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The next evolutionary step was the jellyfish, which had two layers of
cells separated by a gelatinous substance, giving the saucerlike body a means
of support. Unlike the cells of the sponges, those of the jellyfish were inca-
pable of independent survival if separated from the main body. A primitive
nervous system linked the cells and caused them to contract in unison, thereby
providing the first simple muscles used for locomotion. Because jellyfish
lacked hard body parts, they are rare as fossils and usually preserved only as
carbonized films or impressions.
The development of muscles and other rudimentary organs, including
sense organs and a central nervous system to process the information, followed
the evolution of primitive segmented worms. The annelids are segmented
worms with a body characterized by a repetition of similar parts in a long
series. They include marine worms, earthworms, flatworms, and leeches. The
annelids ranged from the upper Precambrian to the present. Their fossils are
rare and consist mostly of tubes, tiny teeth, and jaws.
Since they were bottom dwellers, the early worms left behind a pre-
ponderance of fossilized tracks, trails, and burrows (Fig. 32) to such an extent
that the Proterozoic is referred to as the “age of worms.” Before about 670
Figure 32Fossil worm
borings in Heiser
Sandstone, Pensacola
Mountains, Antarctica.
(Photo by D. L. Schmidt,
courtesy USGS)49PROTEROZOIC METAZOANS