Sponges, Coelenterates, and Worms
How numerous are roundworms?
One soil nematode, Caenorhabditis elegans, is widely cultured and has become a model research organism in developmental biology. The study of this animal was begun in 1963 by Sydney Brenner (1927–), who received the Nobel Prize in Physiology or Medicine in 2002. The species normally lives in soil but is easily grown in the laboratory in Petri dishes. It is only about 0.06 inch (1.5 millimeters) long, has a simple, transparent body consisting of only 959 cells, and grows from zygote to mature adult in only three and a half days. The genome (genetic material) of C. elegans, consisting of 14,000 genes, was the first animal genome to be completely mapped and sequenced.
The small transparent body of this nematode allows researchers to locate cells in which a specific, developmentally important gene is active. These cells show up as bright green spots in a photograph because they have been genetically engineered to produce a green fluorescent protein known as GFP. The complete “wiring diagram” of its nervous system is known, including all the neurons and all connections between them. Much of the knowledge of nematode genetics and development gained from the study of C. elegans is transferable to the study of other animals.
Roundworms, or nematodes, are members of the phylum Nematoda (from the Greek term nematos, meaning “thread”) and are numerous in two respects: 1) number of known and potential species; and 2) the total number of these organisms in a habitat. Approximately 12,000 species of nematodes have been named, but it has been estimated that if all species were known, the number would be closer to 500,000. Nematodes live in a variety of habitats ranging from the sea to soil. Six cubic inches (100 cubic centimeters) of soil may contain several thousand nematodes. A square yard (0.85 square meter) of woodland or agricultural soil may contain several million of them. Good topsoil may contain billions of nematodes per acre.