Final answer:
The two classes in Phylum Nematoda are not well defined, but tentatively include Chromadorea. Nematodes are pseudocoelomates with a complete digestive system and molting cuticle. The phylum has both parasitic and free-living species, such as Ascaris and Caenorhabditis elegans.
Step-by-step explanation:
The two classes included in Phylum Nematoda are not clearly defined due to the ongoing debates and uncertainties in the taxonomy of parasitic worms. However, traditionally Nematoda has been tentatively divided into two classes, including the Chromadorea. The phylum Nematoda comprises over 28,000 species, with around 16,000 being parasitic. Nematodes, or roundworms, are characterized by being pseudocoelomates and having a complete digestive system, a differentiated nervous system, and an elementary excretory system. They also possess a collagenous/chitinous cuticle that requires molting periodically. Additionally, they are bilaterally symmetrical, triploblastic, and exhibit a tubular morphology with a circular cross-section. There are both dioecious and hermaphroditic species in this phylum, and embryonic development proceeds through several larval stages. Examples of parasitic nematodes include Ascaris spp., while a well-known free-living nematode is Caenorhabditis elegans.