ADENOPHOREA

The Adenophorea (also called the Aphasmida) seems to be the most primitive group of nematodes. Mainly, they are free-living in soil and water; however, there are a few parasitic forms of aphasmids. As the alternate name implies, they do not have phasmids (text with tooltip) A phasmid is a sensory organ of parasitic nematodes. , and the amphids (text with tooltip) Amphids (n.) are chemosensory organs at the anterior end of some Nematoda. are located posteriorly on the head region. In fact, they have no sensory bristles or papillae on the head and body. They are simple, spindle-shaped worms with simple excretory organs (single-celled).

• Several taxa of economic importance include Trichinella (Trichina Worm, Figure 4; see the Life Cycle of Trichinella) and Trichuris (Whipworm, Figure 5; see the Life Cycle of Trichuris). Both of them have relatively simple life cycles. Trichinella lives as small adults in the gut. Females brood the eggs and L1 larvae in their bodies and release them after one molt as L2 larvae that burrow into the intestinal mucosa, get into the circulatory system and travel to the tissue where they encapsulate as an L3 larva. Mostly, they inhabit striated muscle (Figure 3), especially the diaphragm. However, they can be found in almost all tissue, including the brain. They may even travel across the placenta and infect a developing fetus. Usually, in humans the cycle ends there, but in animals that practice cannibalism or prey on mammals that might be infect, the L3 larva travels through the stomach and emerges from its cyst in the small intestine. There, they molt and copulate.

SECERENTEA

Secerentea (Phasmida) is a monophyletic group defined by the presence of phasmids. Amphids are located anteriorly in the head region. The excretory system is more complex and includes collecting ducts. Males very often have setae or papillae on the tail, where they also might have lateral extensions. Members of this group include some of the most destructive parasites and include Ascaris, Necator (Hookworm), Dracunculus (Guinea Worm), and Wuchereria (Microfilaria).

• Ascaris is an intestinal parasite that can grow to 50 cm. The female continually releases eggs that pass out of the digestive tract with the feces. The eggs can survive extreme conditions, including those of a sewage treatment plant. Indeed, when I was a graduate student, someone in our group was diagnosed with Ascariasis, which she likely contracted by handling preserved animals. Furthermore, when human feces are used as fertilizer, the incidence of Ascariasis in the community can approach 100%. The eggs have a mucilaginous outer covering which can stick to developing plants. If the food being grown is eaten raw, the eggs go through the stomach, hatch, and the L1 larva emerges. It burrows through the gut wall, gets into the circulatory system and travels to the lungs where it molts and grows in the alveolar space. It molts again and induces the host to cough. The larva is thus brought up to the throat and swallowed. Back in the small intestine, the larva molts once more and grows. Infestations can be very high (Figure 6). Noble and Noble (1976) report that one man had as many as 1,488 worms when he was autopsied. See the Life Cycle of Ascaris
• Necator, the Hookworm, as a mature adult has three razor-like lips with which it slices the intestinal mucosa and pulls the bleeding portion into its buccal area (Figure 7). In that way it feeds on the blood as it oozes from the injured mucosa. Females release eggs which are carried out of the body with the fecal material and hatch on the ground. The larvae are free-living and molt twice to become infective L3 larvae, which burrow into the skin of a host and then travel to the lungs to complete the lifecycle in the same way as Ascaris. See the Life Cycle of Necator
• Dracunculus (the Guinea Worm) lives just under the skin and can grow to a length of more than a meter. The female broods the young in her body and when a host steps into water, she sticks her posterior end out through a hole that she maintains as an open sore. She can expel hundreds of larvae which infect microcrustacea where they live in the haemocel and molt to become infective. The primary host acquires a Guinea Worm by drinking water and ingesting an infected microcrustacean. Larvae burrow through the intestinal wall and develop in the connective tissue where copulation occurs. Males die and the females migrate to the subcutaneous tissue of a lower extremity to complete the cycle. The worms are removed by putting the leg into water and catching the worm as it emerges to release larvae. Once caught, the worm is attached to something like a matchstick and slowly rolled up until it is removed (see Figure 8). See the Life Cycle of Dracunculus
• Filarial worms (Figure 9), like Brugia, live their lives in the lymphatic system. Larvae are released and travel into the peripheral blood, where they may be taken by a mosquito vector. They mature in the mosquito and infective larvae then are injected into another host when she takes another blood meal. Filiariae can pack lymph nodes and cause fluid to pool in the extremities to produce a condition called elephantiasis. See the Life Cycle of Brugia

As debilitating as some of these worms might be, their greatest impact is on the loss of agricultural production (livestock and plants) which have been infected. Whole crops like tomatoes can succumb completely to a heavy infestation in which infective larvae penetrate the roots and cause galls to develop which leads to stunting, low yield, and early die off. Similar problems occur in corn, wheat and rice, all major sources of calories for humanity. Even large trees, like pecans, are plagued by certain nematode parasites.

Most nematodes likely are free-living and frequently seen in water and soil samples in high densities. Certain fungi take advantage of this source of food and produce loop traps in their mycelia (text with tooltip) A mycelium (mycelia, pl.) is a mass of hyphae or fungus-like filaments. . Upon sticking its head through the loop, a nematode will find itself caught by the sudden swelling of the fungal filaments (Figure 10). The fungus then feeds on the dying worm.