Roundworms are pseudocoelomates that belong to the phylum Nematoda. This extremely diverse phylum includes some highly beneficial free-living soil worms, as well as some notorious pests and parasites; the members of this group have very diverse ecologies. The fluid-filled pseudocoelom of these animals can function as a hydrostatic skeleton, which can be useful in drilling through soil or a host's body tissue. Another group that we will study, Annelids, also have hydrostatic skeletons. Rotifers are pseudocoelomates that are an important part of the zooplankton.
Giant kidney worms are able to parasitize many mammals, including humans. The female worms can grow up to 5 feet long. The worms live in the kidney of a vertebrate host, and their eggs leave the body in the host's urine. From there, the eggs might be consumed by an aquatic worm (from the phylum Annelida). Juvenile kidney worms can be transferred to fish or amphibians; if these secondary hosts are consumed by a mammal, the infection cycle is completed.
Given how large these worms can become, it is not surprising that these parasites can cause extensive damage. Note how shrunken the asterisked kidney is, and also note the worms occupying the kidney in the yellow square. The accompanying loss of kidney function can be deadly if the parasites are not detected and surgically removed.
It is estimated that 25% of the world's human population is infected with these roundworms (Ascaris lumbricoides). This particular worm was passed from the intestine of a human. The ruler at the side is approximately 1.5 inches long.
Other nematodes include pinworms, hookworms,
and Trichinella spiralis (the causative agent of
trichinosis). This hookworm mouth reveals how these worms are able to
latch on to a host and penetrate the host's intestines and other organs.
Symptoms of helminth infections include diarrhea, anemia, pneumonia, as well
as physical and mental developmental delays. The World Health Organization
(WHO) data suggest that 1/3 of the world’s population is at risk of infection,
with one in six people already infected.
Caenorhabditis elegans is a microscopic nematode worm that has been used extensively as a model organism in biology. In the laboratory they are an ideal model organism because they can be maintained in a Petri dish, they eat bacteria, and they can be frozen for extended periods, and when thawed, they are completely viable. They have a clear cuticle through which all of their cells can be seen. All adult C. elegans have exactly 959 somatic cells and the individual developmental path of each of these cells has been mapped.
C. elegans is useful in genetic studies because it has a relatively small genome with 5 autosomes and one pair of sex chromosomes (adult C. elegans are either hermaphrodites (XX) or males (X)). It is also relatively easy to disrupt the action of individual genes using a procedure known as RNA interference. In this process, the action of individual genes can be “turned off” by using small pieces of RNA (known as small interfering RNA or siRNA) to degrade a complementary piece of mRNA. By “turning off” a gene, researchers can learn more about the function of that gene.