FURTHER PHYLOGENETIC CONSIDERATIONS
The aschelminths are diverse animals. They have common cuticle, pseudocoelom, muscular pharynx and adhesive glands. But no distinctive features in every phylum.
1. Origin of Rotifers
The rotifers and acoelomates have certain common features. The protonephridia of rotifers closely resemble with some of the freshwater turbellarians. The zoologists giznerally believed that rotifers originated in freshwater habitats. Both flatworms and rotifers have separate ovaries and vitellaria. Rotifers may have originated from the curliest acoelomates. Both these groups may have common bilateral, metazoan ancestor.
2. Origin of other phyla
The kinorhynchs, acanthocephalans, loricIferans and priapulids all have a spiny anterior end. This end can be retracted.Therefore, they are related to each other. Lioriciferans and kinorhynchs are most closely related.
3. Origin and affinities of Nematoda
The affinities of the nematodes with other phyla are not clear. No other living group is closely related to these worms. Nematodes evolved in freshwater habitats. Then they established in the oceans and soils. The ancestral nematodes were sessile. They were attached at the posterior end. The anterior end protrude upward into the water. The nematode cuticle, feeding structures, and food habits have adapted these worms for parasitism. Therefore, free-living species can become parasitic without anatomical or physiological changes.
Nematomorphs are more closely related to nematodes due to cylindrical shape, cuticle, doecious and sexually dimorphic. But the larval form of some nematomorphs resembles plapulids. Therefore, their exact affinity to the nematodes is questionable.