*Phylum Nematoda consists completely of Roundworms.
*Two types: parasitic and free-living. 90,000 species known; many more exist!
*These Roundworms are unsegmented; they have long, narrow, cylindrical bodies that taper to a fine tip in the posterior and a blunt end at the head. Also, they have a cuticle--a tough layer of outerskin--covering their entire bodies.
*They are pseudocoelomates. They have a pseudocoelom, an internal body space surrounding the endoderm tissue of the digestive tract and surrounded by the mesoderm tissue.
*Lacking a circulatory system, nutrients are transported throughout the body via fluid in the pseudocoelom in Roundworms.
*Have a complete alimentary canal with a separate mouth and anus; it transverses the entire length of the organism. This distinguishes them from organisms in the phylum Platyhelminthes, which have only an incomplete digestive tract. [In other words, Roundworms are cooler than Flatworms].
*Longitudinal muscles; they move side-to-side in a thrash-like motion.
Fertilization = internal, and females deposit more than 100,000 fertilized eggs per day. Zygotes can survive harsh conditions.
*Many live in moist soil and in organic matter on the bottoms of lakes and oceans. Play an important role in decomposition and nutrient cycling.
*Closely related to phylum Rotifera; they are both VERY small (microscopic) and have a complete digestive tract.