"Nematodes have many species with relatively little difference in body form. Many are parasitic and it is thought that most species of vertebrates may have one or more parasitic nematode species unique to them. Nematodes parasitize many other groups of animals and plants. Many live in the intestines of animals. One free-living nematode species lives in organic rich soil but can also live as a parasite in humans. Rotting organic matter in soil is not so different from the intestinal contents of some animals. Both are rich in bacteria that the nematodes can feed upon. Adapting to the rich soil made them somewhat "pre-adapted" to life as an intestinal parasite. This adaptation included an ability to live in environments with oxygen so limited many other animals could not survive." (from my unpublished 2010 manscript)
The similarity of structure of different species disappears when the mouth end, and often the anal end, are examined microscopically. Three jaws are present in some. The pharanyx may have a muscular bulb that probably helps ingestion of food without losing pressure, the body contents act as a hydrostatic skeleton. The cuticular covering of the body is molted or shed typically several times in early development. During the process of development portions of the chromosomal material can be ejected from the chromosomes; this is perhaps a result of selection for the small size of ancestors living among the sand grains of soils. In one species the ejection of chromatin occurs in all cells except the stem cell until the 32 cell stage. Body cells of many achelminths other than nematodes also seem to have the loss of ability to regenerate that is thought to be a result of the reduced chromosomal material in body cell nuclei (or nuclei when tissues are syncytial). Near constant number of nuclei or cells of the species are present in the tissues of many aschelminths.
I was reviewing some of Libbie Hyman's work on Aschelminthes (not accepted as a valid cluster by many zoologists), but unfortunately she did not have the benefit of knowing about gnathostomulids (first described in 1956) which were later. Gnathostomulids seem to be descendents of the simple early flatworms that are not flat, but are adapted to living in sediments that are often anoxic. Reidel, 1969, suggests the gnathostomulids can be placed in either the Platyhelminthes or the Aschelminthes. The gastrotrichs may be the connecting link to rotifers. Nematodes may have been the termination of a line orginating early in the cluster of achelminth groups; but they have a complete lack of cilia, a fact that makes them unlikely to have given rise to any other groups since arthropods also lack cilia but are so clearly derived from annelids that do have cilia. Thus the lack of cilia in nematodes and arthropods is an analagous, not homologous, trait.
The reason I referred to Hyman was to find out about the adhesive glands or pedal glands, commonly paired on most ashelminths' posteriors, but absent in the gnathostomulids. The glands are very small and difficult to see, especially in nematodes. I did not see them in some nematodes I had watched in water on a microscope slide at low magnification, but those nematodes were clearly adhering by their tail as the writhed around. One researcher (Chitwood) divided nematodes into two groups depending on whether they had phasmids at their posterior. The mouth area and anal areas of nematodes show great variation in microcopic details not conducive to casual observation.
Such fine details can be a great help in identifying species and often show revealing variation suitable for showing evolutionary relationships. The October 20, 2017, issue of Science has a research report detailing such a fact with feather-like hairs on water-strider feet. In the case of water-striders, the details are limited to very close relatives. In arthropods, similar microscopic comparisons can be made of structures limited to closely related species of the same genus and sometimes of different orders.
When the very small and the very large features match, relationship seems more likely. To determine evolutionary relationships, neither can be ignored. Over-dependence on one may lead to error and demonstrate why the novice or student may see things the specialist or teacher does not see, a relationship affecting creativity as noted by Tinbergen.
Among the larger features distinctive for nematodes, that show them as a terminal group in an evolutionary sense, are the muscle cells of the body of the intestinal parasite, Ascaris. All are longitudinal and each passes a muscle cell process to the nerve enervating the muscle. Other lines of evidence that the Ecdysozoa are an invalid group are indicated by some of the references appended.
The great variations in size, number of host species needed to complete life cycles, and adaption to a single or limited number of final hosts of most vertebrates, as well as many invertebrates, seems to indicate an ancient origin for nematodes.
Joseph G. Engemann Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan October 30, 2017
REFERENCES (comments added)