ZOOLOGY: A STUDY GUIDE - Ye. O. Nevedomska - 2013

LECTURE 9. PHYLUM NEMATODA, OR ROUNDWORMS (Nemathelminthes)

3. Class Nematoda, or Roundworms (Nematoda)

More than 300 species of nematodes have been found in the freshwaters of Ukraine, and about 200 in the Black and Azov seas. Many of them—around 7,000 species—are parasites of humans, animals, and plants. Nematodes range in size from dwarfs to giants. For instance, the length of the free-living marine nematode Trichodermo does not exceed 80 µm, whereas the sperm whale parasite Placentoderma can exceed 8 m in length.

The body of nematodes is typically elongated, thread-like, or fusiform (spindle-shaped), tapering at both ends. This body shape facilitates their movement through highly diverse environments. The anterior end bears a HEAD capsule with a Mouth opening surrounded by Lips. It also features tactile Organs (tangoreceptors) in the form of bristles and papillae, as well as olfactory pits, or amphids.

A well-developed cuticle plays a crucial role, serving both protective and locomotory Functions. Its protective function lies in acting as a barrier that shields the animal's internal Tissues from the destructive effects of various chemicals. This is particularly vital for nematodes that parasitize the digestive tracts of animals and humans. The cuticle is also an essential element of The Musculoskeletal System. Nematode movement results from the antagonistic interaction between the musculature and the cuticle, which is maintained under tension

by the high hydrostatic pressure of the pseudocoelomic fluid. Contraction of the Muscles on one side causes the body to bend, while its straightening is driven by the elastic tension of the cuticle.

The Class of Roundworms includes the large human roundworm (Ascaris lumbricoides), pig roundworm, horse roundworm, pinworm (Enterobius vermicularis), whipworm (Trichuris trichiura), trichina worm, stem nematode, and strawberry nematode.

The human roundworm has an elongated, vermiform, unsegmented, circular in cross-section (Fig. 27), triploblastic body.

Fig. 27. Cross-section through the body of a female roundworm (after Dogiel, 1981):

1 — dorsal nerve cord; 2 — cuticle; 3 — hypodermis; 4 — intestine; 5 — Ovary;

6 — oviduct; 7 — muscles; 8 — Uterus; 9 — longitudinal muscles; 10 — ventral nerve cord;

11 — excretory canal; 12 — rachis (support cords); 13 — Ovaries.

The body length is 20–40 cm, and the color is yellowish-white. The anterior end features a mouth with two lips. The Digestive System is represented by a primitive tube divided into three sections: the foregut (beginning with the mouth), the midgut (intestine), and the hindgut, which terminates in the anus. Digestion occurs within the intestinal lumen. The roundworm lacks a Respiratory system and is an obligate anaerobe.

The human roundworm possesses several adaptations to a parasitic lifestyle. Its body is resilient, elongated, cylindrical, and tapered at both ends, allowing it to move easily through the intestines. Lacking attachment organs, the roundworm must constantly crawl against the flow of food masses or brace itself against the intestinal walls using its elastic body. If the intestinal contents sweep it along, the roundworm would be expelled outward with the waste. The cuticle of the roundworm secretes a substance—keratin—that neutralizes the action of the host's main digestive Enzymes, Pepsin and Trypsin. In free-living nematodes, keratin is either entirely absent from the cuticle or present in negligible amounts. At the same time, the cuticle is highly permeable to glucose, potassium iodide, sodium, and other substances derived from the host's body.

Roundworms are dioecious animals (Fig. 28) exhibiting Sexual Dimorphism: females measure 25–40 cm, while males are 15–20 cm long. In the male, the posterior end of the body is slightly broadened and curved ventrally into a hook; the female possesses two long, thin ovaries, oviducts, and uteri, as well as a vulva; the male features a single thread-like Testis, vas deferens, and ejaculatory duct. Fertilization is internal. Roundworms reproduce via eggs.

Fig. 28. Human roundworm (after Dogiel, 1981):

a — female:

1 — lips; 2 — Esophagus; 3 — intestine; 4 — genital pore (vulva); 5 — uterus; 6 — oviduct; 7 — ovary;

8 — muscles; 9 — lateral line; 10 — anus;

b — Male Reproductive System:

1 — vas deferens; 2 — testis; 3 — ejaculatory duct; 4 — hindgut.

Each mature female produces up to 240,000 eggs per day. According to rough estimates, the total mass of eggs produced by a female over her lifespan of about one year exceeds her own body mass by 1,700 times. The eggs are released in an unsegmented, undeveloped state. For a larva to develop from the egg, oxygen, adequate humidity, and warmth are required (Scheme 5).

Scheme 5. Life Cycle of the large human roundworm.

Under favorable conditions, egg maturation takes two weeks, while in moist soil or Water they can survive for 5–6 or even 10 years while remaining infective. The fertilized egg is coated with four membranes that reliably protect it from adverse environmental conditions. In the human intestine, the ascaris larva sheds its membranes and penetrates the intestinal wall into the bloodstream. Carried by the Blood flow, the larvae are transported to the Lungs, where they temporarily localize in the alveoli. Here they develop over 7–10 days, molting three times. Afterward, they leave the lungs and migrate up the Bronchi into the Trachea, then into the Pharynx, and from there reach the Oral Cavity. Swallowed along with saliva, the larvae re-enter the intestine, where they mature into adult parasites. Research has established that it takes 89 days for a larva to develop into a sexually mature Organism in The Human Body (see Scheme 5). Preventive measures against ascariasis and its control are presented in Table 7.

Table 7

HUMAN ROUNDWORMS — PARASITIC NEMATODES AND THEIR CONTROL

The pinworm inhabits the human small and large intestines, causing the disease enterobiasis. This parasite is widespread across all countries and geographical latitudes, occurring most frequently in children. The pinworm's body is spindle-shaped, with its anterior end featuring a cuticular expansion known as a vesicle. The female is 10–13 mm long, and the male is 8–11 mm long. The posterior end of the female's body is elongated and pointed, whereas in the male it is spirally curved. After fertilization, the males die, while the females crawl out through the anus and deposit eggs (about 12,000) on the Skin folds, after which they also die. The eggs mature within 4–6 hours, facilitated by optimal conditions: favorable Temperature, oxygen availability, and moisture. When laying eggs, the female releases a secretion onto the skin that causes itching. Scratching leads to the accumulation of numerous eggs under the fingernails and on the skin of the fingers. The lifespan of a pinworm is 4–6 weeks. Children infected with pinworms become restless, Sleep poorly, experience a loss of appetite, and suffer from abdominal pain. Additionally, pinworms can be a cause of appendicitis.