“Section of female Ascaris photographed with Nikon 1,40 Darkfield oil condenser. 200 magnification. The large circles filled with small green circles are the uterus and eggs. The long narrow feature is the digestive tract. The smaller red and orange circles are the ovaries and oviducts. The cluster of green and black blobs in the upper right and lower left are the nerve cords (ventral and dorsal). Surrounding the internal organs are the frilly green longitudinal muscles, the dark hypodermis, and the green outer cuticle.” - via Wikimedia Commons
Nematoda is a diverse phylum of animals commonly called roundworms or eelworms. Most are free-living and feed on microorganisms, but some are parasitic.
Nematodes are very diverse, but usually appear as small, slender worms. The smallest are microscopic, while the largest free-living species can be up to 5 cm (2 in) long. Some parasitic species can be even longer, reaching up to 8.4 m (27.5 ft) in length! Nematode heads are radially symmetrical and, in many cases, have head-shields radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edges. They have a dense, circular nerve ring which serves as their brain. They are covered in sensory bristles that provide a sense of touch. There are two small pits on the head that likely serve as chemoreceptors. Some aquatic nematodes have eye-spots, but it is unknown if they are actually sensory. They have seperate male and female individuals, with females usually being larger than males, though some species are hermaphroditic. They reproduce sexually, and females have a glandular uterus. They lay eggs, though some species are ovoviviparous. Larvae of free-living nematodes look like smaller adults, though parasitic nematodes usually have more complex life cycles.
Nematodes are perhaps the most successful phylum on Earth. They have adapted to nearly every ecosystem: from marine to freshwater, from soils to trees, from tundra to rainforest, at the tops of mountains, in deserts, in oceanic trenches, and up to 3.6 km (12,000 ft) below the surface of the Earth. They represent 90% of all animals on the ocean floor, and 80% of all individual animals on Earth. They often exceed a million individuals per square meter. This ubiquitous nature means they play a role in every ecosystem, most crucially in polar ecosystems where life is otherwise scarce. Of the parasitic forms, about a third of genera occur as parasites of vertebrates, and about 35 nematode species occur in humans.
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Nematodes play an important role in the decomposition process, aiding in recycling nutrients.
There are approximately 60 billion nematodes per human inhabiting the Earth’s topsoil. Those are your government assigned topsoil nematodes. Do with them as you wish.
One soil-living nematode, Caenorhabditis elegans (see first image), has had its entire genome sequenced, the developmental fate of every cell determined, and every neuron mapped. They are considered a model organism: a non-human species that is extensively studied to understand particular biological phenomena.
While some species of nematode are detrimental to agriculture, other species are considered beneficial as they prey on agricultural pests. These species are bred commercially as biological pest control agents which can be used as a much safer, environmentally-friendly alternative to pesticides.
The largest known nematode, Placentonema gigantissima, can reach sizes of up to 8.4 m (27.5 ft) long and 2.5 cm wide. It has been found living as a parasite in the reproductive tract of a sperm whale.
As stated by nematologist Nathan Cobb:
“In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable since, for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.”
Annelida: Segmented Worms. This group includes earthworms, leeches, and many classes under the umbrella of "polychaete". This diverse phylum encompasses deposit feeders (eating dirt), detritivores, scavengers, deadly ambush predators, filter feeders, parasites, herbivores, and more. They are broadly defined by their repeating body segments and parapodia, which are nubby appendages used for both movement and breathing. Some have curved jaws for catching prey or scraping detritus off of rocks, while others have wide, elaborate feather-like fans for filter feeding. While able to crawl freely, a majority of marine Annelids spend most of their time in self-built tubes or burrows. Among their many important functions, they play a key role in mixing soil/sediment, breaking down decaying organic matter, and providing a key food source to countless other animals.
Nematoda: Roundworms. Split between free-living and parasitic, terrestrial and aquatic, Nematodes inhabit just about every environment on Earth. Nematologist Nathan Cobb once said (paraphrased) that if all matter on Earth disappeared besides Nematodes, we would still be able to identify where everything used to be, simply by the thin layer of nematodes left behind. In the harsh environment of Antarctica's Dry Valleys, these worms dominate the relatively barren ecosystem. The roundworm C. elegans is a widely-studied model organism in science and medicine. Nematodes are also known to parasitize humans, due to their ability to enter a dormant larval state within the muscle of a carrier animal. This is part of why we fully cook pork; to kill any parasites in the meat.
