Chapter 15: Phylum Nematoda: The Roundworms
Roundworms
The common name for phylum Nematoda is roundworms. They are among the most numerous of all animals. A single rotting apple can contain as many as 90,000 of them, and over a million can be found in a cubic meter of garden soil. Roundworms are pseudocoelomates (“pseudo” = false), because they do not have a true coelom. This means that they do have the peritoneal cavity (or gut), but it is not lined with mesoderm.
Nematode = Thread?
Roundworms were given the name nematodes because they resemble a thread. In Greek, ”nematos” actually means thread, hence why they are called nematodes. About 12,000 species have been named, but estimates say that if we knew about all the different species, that number would be closer to 500,000.
What is a Roundworm?
Roundworms are slender, unsegmented worms with tapering ends. They can be microscopic or up to a meter in length. Most species of roundworms are free-living, inhabiting soil, salt flats, aquatic sediments, and water from polar to tropical regions. Still others are parasitic and live in hosts that include almost every kind of plant and animal.
What is a Roundworm?
Nematodes parasitize virtually every type of animal and many plants. The effects of nematode infestation on crops, domestic animals, and humans make this phylum one of the most important of all parasitic animal groups. Almost all species of vertebrates and many invertebrates serve as hosts for one or more types of parasitic nematodes.
What is a Roundworm?
Unlike the platyhelminthes, nematodes have a digestive tract with two openings. This body plan is often called a “tube-within-atube.” The outer tube is the body wall and the inner tube is the digestive tract. This arrangement makes digestion in roundworms very different from that in flatworms because food moves in one direction through the digestive tract.
Form and Function in Roundworms
Roundworms have specialized tissues and organ systems that carry out essential body function. In general, the body systems of free-living roundworms tend to be more complex than those of parasitic forms. Distinguishing characteristics of this phylum are their cylindrical shape, flexible nonliving cuticle, lack of motile cilia or flagella, lack of protonephridia, and the muscles of their body wall run only longitudinally.
Body Covering
The outer body covering is a relatively thick, noncellular cuticle secreted by the underlying epidermis, or hypodermis. The several layers of the cuticle are primarily of collagen, a structural protein also abundant in vertebrate connective tissue.
Feeding
Many free-living roundworms are carnivores that use grasping mouthparts and spines to catch and eat other small animals. Some soil-dwelling and aquatic forms eat algae, fungi, or pieces of decaying matter. Other nematodes digest the bacteria and fungi that break down dead animals and plants.
Respiration, Circulation, and Excretion
Like flatworms, roundworms exchange gases (respire) and excrete metabolic wastes like urea and ammonia through their body walls. They have no internal transport system. Diffusion is the main process through which nutrients and wastes are carried through their bodies.
Response
Nematodes have simple nervous systems, consisting of several ganglia. Several nerves extend from ganglia in the head and run the length of the body. These nerves transmit sensory information and control movement. Roundworms have several types of sense organs.
Response
A ring of nerve tissue and ganglia are found at the anterior end of their bodies. They have a pair of amphids that are somewhat more complex sense organs that open around their heads. They also have a pair of phasmids that are similar in structure as amphids, but open around the posterior end of the body.
Movement
The muscles of nematodes extend the length of their bodies. Together with the fluid in the pseudocoelom, these muscles function as a “hydrostatic skeleton.” A hydrostatic skeleton is the use of coelom fluid to maintain the shape of the animal and allows for locomotion. Aquatic roundworms contract these muscles to move like snakes through the water. Soil-dwelling roundworms push their way through the soil by thrashing around.
Muscular System
Body wall muscles of nematodes are very unusual. They have no circular muscles. Each muscle cell has a contractile fibrillar portion (composed of fibers), and a noncontractile sarcoplasmic (clear, semifluid cytoplasm between the fibers in the muscle fibers) portion.
Reproduction
Roundworms reproduce sexually, and most species of roundworms are dioecious. They reproduce using internal fertilization: the male usually deposits sperm inside the female’s reproductive tract. Parasitic nematodes often have complex life cycles that involve two or three different hosts or several organs within a host.
Classes of Nematoda
There are two main classes of nematodes: Rhabditea and Enoplea.
Class Rhabditea – Amphids are ventrally coiled or derived therefrom; three esophageal glands; some with phasmids; there are both free-living and parasitic forms. Examples: Caenorhabditis, Ascaris, Enterobius, Necator, Wucheria Class Enoplea – Amphids usually well-developed, pocket-like; five or more esophageal glands; phasmids absent; excretory system lacking lateral canals; formed of single, ventral, glandular cells, or entirely absent; mostly free living, but includes some parasites. Examples: Dioctophyme, Trichinella, Trichuris.
Caenorhabditis elegans
In 1963 Syndey Brenner started studying Caenorhabditis elegans, a free-living nematode. It has turned out to be one of the most important experimental models in biology. It is easy to grow in the laboratory, and we have been able to map the genome and the “wiring diagram” of the nervous system of this roundworm.
Nematode Parasites
Many nematodes are very important pathogens of humans and domestic animals. Some of the nematodes we will discuss:
Ascaris lumbricoides Hookworms Trichina Worm Pinworms Filarial Worms
Ascaris lumbricoides
Ascaris lumbricoides occurs in up to 64% of people in some areas of the southeastern U.S. More than 1.2 billion are affected worldwide. A. megalocephala is found in the intestine of horses. A. suum is found in pig intestines.
Ascaris lumbricoides
A female Ascaris can lay 200,000 eggs per day, passing out through the host’s feces. In good soil, embryonation is complete in two weeks. Viable eggs remain after signs of fecal matter have disappeared. Eggs can survive long periods in the soil. Barefoot? I think not.
Ascaris lumbricoides
When a host swallows the eggs, juveniles hatch and burrow through the intestinal wall. The juveniles then are carried through the heart to the lungs. When at the lungs, they break into the alveoli and are carried up to the trachea. Juveniles are coughed up and swallowed, then mature in the intestine two months after they were swallowed. They feed on intestinal contents and may block or perforate the intestines.
Hookworms
Hookworms are so named because the anterior (head) end curves dorsally, resembling a hook. They are dioecious. Necator americanus is most common species. They have large plates in their mouths that cut into the intestines so that they can suck on the host’s blood.
Hookworms
Hookworms pump more blood than they can digest. A heavy infection can cause anemia. Eggs pass in feces and juveniles hatch in soil where they can live off of bacteria. If human skin comes in contact with the soil, infective juveniles burrow through the skin to blood. Their life cycle is similar to that of Ascaris.
Trichina Worm
Trichinella spiralis is tiny, but causes a potentially lethal trichinosis. Adult worms burrow into the intestines and females directly produce living young. The juveniles penetrate blood vessels and circulate throughout the body to all tissues and spaces.
Trichina Worm
Juveniles penetrate skeletal muscle cells, change its gene expression so that it becomes a nurse cell to the parasite. When meat containing live juveniles is eaten, worms are liberated and mature in the intestine.
Trichina Worm
They infect humans, hogs, rats, cats, and dogs. Hogs can become infected eating uncooked scraps of infected meat or rats. There are four other sibling species with variable distribution, freezing resistance, etc. Heavy infections can cause death but lighter infections are more common. About 2.4% of the U.S. population is infected, mostly lightly.
Pinworms
Pinworms are the most common worm parasite in the U.S., but causes little disease. It is estimated that 30% of children and 16% of adults in the U.S. have them. Adults live in the large intestine and cecum.
Pinworms
Females, about 12 mm in length, migrate to the anal region at night and lay eggs, causing itching. Scratching the anal region contaminates hands and bedclothes. So how do you test for something like this?
Scotch Tape Method
Doctors usually diagnose pinworms by fecal examinations and finding the eggs, but eggs are often not found in feces. Many times the female pinworm will deposit her eggs on the skin around the anus. Doctors have started using the “scotch tape method.” Truth is in fact stranger than fiction. See page 316 in your textbook.
Scotch Tape Method
The scotch tape method consists of placing the sticky side of cellulose tape onto the anus overnight. The next morning the tape is umm...harvested and placed under a microscope to search for eggs. Several drugs are effective against it, and all members of the family should be treated at the same time because the worms spread easily through a household.
Pinworms
Eggs develop rapidly and become infective within six hours at body temperature. When swallowed, these eggs hatch in the anterior end of the small intestine (the duodenum) and mature in the large intestine. Members of this order have haploid (one set of chromosomes) males from unfertilized eggs and diploid females from fertilized eggs. This is known as haplodiploidy.
Filarial Worms
There are eight species of filarial nematodes that infect humans. Some cause major and serious diseases. About 250 million people in tropical countries are infected with Wuchereria bancrofti or Brugia malayi, which live in the lymphatic system. They cause inflammation and blockage of the lymphatics. Females can be as long as 100 mm and can release live young, or tiny microfilariae into the blood and lymph.
Filarial Worms
Mosquitoes ingest the microfilariae when they feed. The worms develop to the infective stage while inside the mosquito and move into the mosquito bite wound when it feeds.
Filarial Worm Diseases
Filarial worms cause three main diseases in their definitive hosts:
Elephantiasis River blindness Dog heartworm
Elephantiasis
Elephantiasis symptoms are apparent after long and repeated exposure to filarial worms. It is marked by excessive growth of connective tissue and enormous swelling of affected parts, such as the scrotum, legs, arms, and more rarely the vulva and breasts.
Onchocerciasis: River blindness
River blindness is caused by a filarial worm that is carried by black flies. It infects more than 30 million people in parts of Africa, Arabia, Central America, South America, and virtually all other tropical areas.