Introduction to the "Aschelminth" Phyla

Microscopic nematode (roundworm) photographed with the UCMP Environmental Scanning Electron Microscope

A very large and heterogeneous cluster of animals have traditionally been classified together in a group variously known as the Aschelminthes, Nemathelminthes, and/or Pseudocoelomata. Today, these organisms are classified in about ten separate phyla. Nonetheless, it is sometimes useful to retain the name "Aschelminthes" to cover all these organisms. Opinion is divided as to whether or not the aschelminth phyla form one monophyletic group. To make matters worse, different authors have not agreed on what phyla should be considered "aschelminths."

The most commonly recognized aschelminth phyla are:

Acanthocephala -- spiny-headed parasitic worms; about 1150 species known
Chaetognatha -- arrowworms; about 70 species known.
Cycliophora -- cycliophorans; 1 species known, microscopic
Gastrotricha -- gastrotrichs; about 430 species known, all microscopic
Kinorhyncha -- kinorhynchs; about 150 species known, all microscopic
Loricifera -- loriciferans; about 10 species described, all microscopic
Nematoda -- nematodes or roundworms; about 12,000 species known, but an estimated 200,000+ species extant, mostly microscopic
Nematomorpha -- horsehair worms; about 320 species known
Priapulida -- priapulid worms; 16 species known, abut half microscopic
Rotifera -- rotifers or "wheel animalcules"; about 1500 species known, all microscopic

Of these, probably the most familiar is the Nematoda. Nematodes make up the second most diverse animal phylum, second only to the arthropods. Free-living nematodes are exteremely abundant in soils and sediments, where they feed on bacteria and detritus. Other nematodes are plant parasites and may cause disease in economically important crops. Still others parasitize animals (including humans); well-known parasitic nematodes include hookworms, pinworms, Guinea worm (genus Dracunculus), and intestinal roundworms (genus Ascaris).

So what, if anything, is an aschelminth? Most are soft-bodied worms, and many of them are microscopic -- in fact, practically all members of the Cycliophora, Gastrotricha, Kinorhyncha, Loricifera, and Rotifera are less than 1 millimeter long, as are many nematodes and priapulids. On the other hand, one species of parasitic nematode can reach 13 meters in length -- it parasitizes the sperm whale -- and adult nematomorphs, chaetognaths, and some priapulids are also visible to the naked eye.

Aschelminths used to be referred to as "pseudocoelomates" -- an alternative name for the taxon is Pseudocoelomata -- because of their supposed shared internal structure. True coelomates have a fluid-filled body cavity, the coelom, that surrounds the gut; this cavity exists within the middle tissue layer, the mesoderm, and the gut is suspended within it by sheets of tissue known as mesenteries. "Pseudocoelomates" may also have a body cavity around the gut; in some (e.g. the gastrotrichs) it is extremely small, while in others (e.g. nematodes and priapulids) it may be quite extensive. This cavity has traditionally not been considered a true coelom, because it supposedly does not exist within the mesoderm, true mesenteries are not present, and its development in the embryo is quite different. However, this dichotomy between "coelomates" and "pseudocoelomates" appears to be false. A full discussion of this issue is beyond the scope of this exhibit; suffice it to say that some traditional "coelomates" and some traditional "pseudocoelomates" do not fit their traditional definitions. Body cavities develop in several different ways and perform many different functions within the animal kingdom.

A number of aschelminths are parasitic, including the Acanthocephala, which parasitize vertebrates; the Nematomorpha, which parasitize insects and other arthropods; and the Nematoda, which include parasites of plants and animals as well as many non-parasitic, free-living species. Free-living, microscopic "aschelminths" may be extremely common in moist soils and fresh-water sediments (gastrotrichs, rotifers and nematodes). Others may abound in marine sands and muds (gastrotrichs, kinorhynchs, loriciferans, nematodes, priapulids, rotifers). The arrowworms, found in marine waters, are generally planktonic and can swim; they are voracious predators on other planktonic organisms. Many small aschelminths, in particular many rotifers and nematodes, are able to suspend their life processes completely when conditions become unfavorable; in these resistant states they can survive extreme drying, heat, or cold, and then return to life when favorable conditions return. This is known as cryptobiosis.

Since aschelminths all lack substantial hard parts, their fossil record is extremely spotty. Most of the microscopic phyla have no known fossil record at all, and seem unlikely ever to be found as recognizable fossils. The Rotifera are only known as far back as the Oligocene. Fossil nematodes are occasionally found in amber (fossilized tree resin) from the Cenozoic, but possible fossil nematodes have been found in older rocks of the middle Paleozoic. The oldest known "aschelminth" phylum is the Priapulida. These unsegmented worms have been found in Cambrian rocks, such as the legendary Burgess Shale. Priapulids, at least, date from the earliest known evolutionary radiation of the animals, and a few trace fossils suggest that nematodes may have been around at the time as well. Evidence from the anatomy and genetic information of living "aschelminths" suggest that the aschelminth phyla are much older than their sparse fossils would indicate. However, no consensus has yet been reached as to how these phyla evolved and how they are related to each other.

We don't have an extensive exhibit on these organisms, but we can suggest some places to visit on the Web if you would like more information. An excellent place to start is the International Association of Meiobenthologists, whose members study all manner of tiny creatures within sediments (not just aschelminths); their resources include an electronic bulletin board where scientists can post messages.

Resources for specific taxa include:

Chaetognatha: Some information on arrowworms is available from the Canadian Institute of Ocean Sciences, Plankton Productivity Division.
Gastrotricha: Visit the homepage of Dr. William Hummon, an expert on these organisms.
Nematoda: Perhaps the best starting point for learning about nematodes on the WWW is the home page of the Society of Nematologists.
Nematomorpha: A report on the horsehair worms of Illinois, with some general information about the phylum, is available from the Illinois Natural History Survey.
Rotifera: See this excellent page on the taxonomy and biology of rotifers.