Parasitic vs. Free Living Worms
Relationships among parasitic Platyhelminthes12http://www.eplantscience.com/index/general_zoology/phylogeny_and_adaptive_radiation03.php
Phylum Nematoda contains species that can affect humans both directly and indirectly through domestic animals, and also free-living species that can be decomposers and predators on microorganisms3. Although there are two types, both species contain similar characteristics that make them parasitic and overall more successful. Just like Phylum Platyhelminthes, the characteristics that make Nematodes special include having a triploblastic and bilateral symmetrical body that is covered with a secreted, flexible, non-living cuticle, and muscles that run longitudinally in the body wall5. Nematodes also lack features such as cilia and a well-defined head6. The sizes of most of the species in this phylum are under 5 cm long, while others can be microscopic5. Parasitic Nematodes, on the other hand, can be as long as 1 meter. Their bodies are long and narrow and have an epidermis that is made of mass of cellular material and nuclei without separate membranes7. Moreover, parasitic nematodes have more than one host to distribute themselves and finish their lifecycle.
Caenorhabditis elegans (C. elegans) is a prime example of a free-living roundworm that primarily lives in soil where it can feed off bacteria and fungi. Advantages to this free-living nematode include its microscopic size, ease of propagation, compact genome, short life cycle, and high fecundity rate2. C. elegans, like the other nematodes, has an unsegmented, cylindrical body shape that is tapered at the end2. These roundworms contain hermaphrodites and male worms. These characteristics thus make C.elegans more successful than typical free-living species.