Nematodes

Members of the Phylum Nematoda are commonly known as "roundworms." The nematodes and rotifers, along with some other minor phyla, are sometimes referred to as the "aschelminths", though this is a loose association no longer used. Most are free-living and can be found in nearly any habitat (from rotting fruit to arctic soils). Some members of this phylum are parasitic, and these have given the phylum a lot of bad press, since they are the animals usually covered. Ascaris lumbricoides is one of the best known of the parasitic nematodes. Males and females live in the intestines of humans where they graze on intestinal contents. Eggs pass out with the feces and, if they contaminate food, are introduced to another host. The larvae hatch out in the intestine of the new host and then burrow through the walls to be carried by the bloodstream to the lungs. At the lungs they burrow through the alveoli and crawl up the trachea and down the esophagus. Larvae usually burrow out of the lungs at night and are unknowingly swallowed by the host. Occasionally larvae get lost and crawl up the esophagus and exit at the nose. In some areas of the world Ascaris is so common that a child is not considered to be part of the tribe until a larva is sneezed out and found in the bed.

Ascaris has a pseudocoelom level of body development (Click HERE for more information on body cavities.) and a complete digestive system starting with a muscular triradiate pharynx, followed by a thin, ribbonlike intestine and terminated by the slitlike anus (Figure 1). When cut in cross section the triradiate pharynx has a Y-shaped opening at its center. This structure increases the suctioning efficiency of the pharynx (Ascaris drink fluids in their host's intestines). Excretory canals (lateral lines) pick up cellular waste and excrete it to the outside at excretory pores (located at the anterior end of the animal). All other structures are related to reproduction. The larger animals with the straight tails are females while thin ones with hooked tails are males. The female's genital pore is a slit-like opening about a third of the way down from the anterior end and connected to a short vagina. The vagina leads to a pair of uteri. At the end of each uterine segment, a thin oviduct can is found, followed by the threadlike ovaries. Eggs are produced by meiosis within the ovaries. They then move along the oviducts to the uterus. While some maturation of the eggs occurs within the oviducts, fertilization takes place in the uterus.

l The anus serves as both the excretory and reproductive pore for male Ascaris. The copulatory spicule is a hook-like appendage used to hold the female's genital pore open and is located near the edge of the anus. At the posterior end of the intestine there is a branch and below the branch is the cloaca (connected to the anus on the outside). The cloaca serves as a common collecting area for fecal material from the intestine and spermatozoa from the seminal vesicle. A short ejaculatory duct is located at the end of the seminal vesicle. Relatively thick tubes attached to the seminal vesicle are the vas deferens which connect to finer threads (the testes). Sperm are produced in the testis and they mature as they move along the vas deferens to the seminal vesicle. During copulation, the sperm enter the cloaca by way of the ejaculatory duct before being deposited in the female. Nematode sperm are unusual because they are amoeboid (not flagellated).

Under cross section the outer cuticle can be seen (Figure 2). The cuticle is resistant to intestinal enzymes and is deposited by the underlying hypodermis. Muscles are arranged longitudinally (they run along the long axis of the body). The absence of circular muscles is partly responsible for the undulatory movements that are characteristic of nematodes. The lateral lines are closely associated with the pseudocoelom and are used to drain metabolic wastes from the body cavity. Except for the intestine, most of the pseudocoelom is occupied by reproductive structures. The uteri are easily recognized because they're filled with eggs. Oviducts can be distinguished from ovaries by the presence of an open canal at the center.

If Intestines of domestic animals (and humans) can be infested with a variety of nematodes including hookworms (Necator americanus), pinworms (Enterobius vermicularis), and whipworms (Trichinella spiralis) may be seen. Hookworm infections can be extremely dangerous (these animals feed on the intestinal walls), while that of the pinworm is mostly an annoyance to parents of young children. Adult whipworms live in the intestinal walls, and their larvae are carried through the circulatory system to skeletal muscles. There the larvae encyst (heavy infestations cause muscle pain and flu-like symptoms we call trichinosis). If the infected meat is eaten, the larvae are passed on to a new host.

Ascaris - Diagram of the cross section of the pig round worm	Cross section of an Ascaris female.	Cross section of the Ascaris female reproductive system showing ovry and oviduct.	Cross section of the Ascaris female reproductive system showing uterus with eggs.
Cross section of the Ascaris male.	Internal anatomy of the female ascarid worm showing, ovary, oviduct uterus and the position of the genital opening.	Comaprison of the posterior end of a male and female ascarid worm	Vinegar eels.
Trichinella Worms in Muscle