Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Multicellular Animals
Roundworms, or Pseudocoelomates
Class Nematodes, or True Roundworms

The body of nematodes is spindle-shaped or thread-like, circular in cross-section, and unsegmented (Fig. 24). The Mouth is located at the anterior end of the body and is often surrounded by Lips (for instance, three in Ascaris). Not far from the posterior end, on the ventral side, lies the anus, behind which is the tail region. Roundworms are covered with a dense layer of elastic cuticle. The features of their internal Structure are shown in Figure 25.

The cuticle has a protective function. In addition, it maintains a sufficiently high pressure (turgor) of the pseudocoelomic fluid. This is precisely what gives nematodes their cord-like elongated shape.

The inner surface of the body wall is lined with the hypodermis, which forms thickenings in the form of 4 ridges running along the body of the worm laterally, as well as along the dorsal and ventral sides. Beneath the hypodermis lie 4 bands of longitudinal Muscles. Upon contraction, the dorsal and ventral bands act as antagonistic muscles, allowing the nematode body to bend. The Skin-muscular sac (cuticle, hypodermis, longitudinal musculature) encloses the primary body cavity, filled with pseudocoelomic fluid, in which the Organs of the digestive and reproductive systems are located.

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Fig. 24 External appearance of Ascaris (A — male, B — female):

1 — mouth opening and lips; 2 — excretory system opening; 3 — female genital pore; 4 — constriction on the female body (clitellum-like region); 5 — female anus; 6 — curled posterior end of the male body; 7 — cloacal opening.

Fig. 25 Cross-section of a female Ascaris body:

1 — cuticle; 2 — hypodermis; 3 — Muscle Cells; 4 — dorsal nerve cord; 5 — ventral nerve cord; 6 — lateral hypodermal ridges with the excretory system canal; 7 — primary body cavity; 8 — intestine; 9 — Ovary; 10 — oviduct; 11 — Uterus.

The Digestive System is represented by a straight tube comprising the mouth, Pharynx, midgut, and hindgut. Food is digested and absorbed in the middle section of the intestine. Undigested waste is eliminated from the worm's body through the hind section, which terminates in the anus.

The excretory system of nematodes is represented by two canals, closed at the posterior end, running within the hypodermal ridges along the sides of the body. Approaching the anterior end of the worm, the canals turn toward the ventral side and merge into a single, unpaired canal that opens on the ventral side via an excretory pore. This entire system consists of a single giant Cell, The Nucleus of which is located in the Cytoplasm of the left canal. In addition to cervical glands (as the excretory organs of roundworms are called), phagocytic cells perform the function of extracting substances unnecessary to the Organism from the coelomic fluid.

The Nervous system of nematodes is located within the hypodermis. It consists of a circumoesophageal nerve ring and several nerve trunks extending from it (there are 6 in Ascaris). Sense Organs are poorly developed, represented only by organs of chemical sense and Touch.

Roundworms are predominantly dioecious. They exhibit clearly pronounced Sexual Dimorphism. In males, the posterior end of the body is curled. Furthermore, females are larger. In the anterior third of their body, a distinct belt-like region is noticeable, on the ventral side of which the genital pore is located. The reproductive organs are paired in females and unpaired in males.

A distinctive feature of nematodes is cellular constancy (eutely). Cell Division ceases at early developmental stages. Consequently, the total number of cells remains constant throughout their entire life.

Among roundworms parasitic in humans—especially children of preschool and early school age—the large roundworm (Ascaris lumbricoides) and the pinworm (Enterobius vermicularis) are widespread. Trichinella (Trichinella spiralis) is a particularly dangerous parasite.

The large roundworm (Ascaris lumbricoides) inhabits the Small Intestine of humans, most commonly children. Adult females reach up to 40 cm in length, and males up to 20 cm. They cause ascariasis, a disease characterized by headaches, general weakness, dizziness, irritability, and decreased work capacity and memory. These disorders in the body's condition are caused by the release of toxic metabolic products of Ascaris into the human intestine. Additionally, tangles of these parasites can cause intestinal obstruction.

Fertilized Ascaris eggs are excreted from the host's body with feces (Fig. 26). In the external environment under optimal conditions—presence of oxygen, air Temperature of 24-25°C, and sufficient humidity—egg maturation takes 2-3 weeks, whereas in the absence of such conditions, development may take longer.

A matured egg is called invasive, i.e., infective, because it contains a fully formed larva. An invasive egg may be swallowed by a person along with unwashed vegetables, fruits, or berries, or enter the mouth from dirty hands. In the intestine, a larva hatches from the egg and actively penetrates the Blood Vessels of the intestinal walls. Carried by the bloodstream, it enters the Liver vessels, from there to the right ventricle of The Heart, and further into the Lungs; moving actively, it passes from the blood into the pulmonary alveoli, Bronchi, Trachea, and into the host's Oral Cavity. Swallowed with saliva or sputum, the larva enters the intestine once again, where it develops into a sexually mature worm.

Fig. 26 Life Cycle of the large roundworm (Ascaris lumbricoides)

Prevention of ascariasis consists in strict observance of personal hygiene rules: washing vegetables, fruits, berries, and hands before eating, as well as implementing public health measures: improving and properly maintaining latrines, conducting health education work, and prohibiting the Fertilization of vegetable and berry gardens with human feces.

The pinworm (Enterobius vermicularis) parasitizes most frequently in children within the lower section of the small intestine, the cecum, and its vermiform Appendix (Fig. 27).

Females are about 10 mm long, and males 3-5 mm. Pinworms cause enterobiasis, a condition characterized by restless Sleep, sleep deprivation, deterioration of well-being, decreased work capacity, and even neurological disorders. Sometimes pinworms can cause inflammation of the vermiform appendix, leading to appendicitis.

Fig. 27 Human pinworm (Enterobius vermicularis):

1 — mouth; 2 — Esophagus; 3 — muscular bulb; 4 — intestine; 5 — ovary; 6 — uterus; 7 — caudal end; 8 — anus; 9 — female genital pore; 10 — cephalic vesicle.

After fertilization, the females migrate toward the human anal opening, crawl out—causing intense itching—and deposit eggs on the perianal folds and buttocks. Within 6 hours of oviposition, the eggs mature and become infective. Being small and readily dispersed, they end up on clothing, furniture, and books, and accumulate in large quantities under fingernails. The eggs of pinworms can be transferred from hands to the mouth and swallowed, resulting in self-infection (autoinfection).

Prevention of enterobiasis relies on strict personal hygiene, including thorough handwashing and keeping fingernails clean. The undergarments of infected individuals must be changed every morning, washed, and ironed with a hot iron. In rooms and classrooms where infected children are present, systematic wet cleaning is essential. An important role in preventing enterobiasis belongs to health education, especially

Trichinella (Trichinella spiralis) spends part of its life cycle in the intestine and another part in the Muscles of the host. Accordingly, two phases are distinguished: intestinal and muscular trichinellae (Fig. 28). The hosts of Trichinella are various carnivorous mammals (predators, rodents, pinnipeds), including humans, in whom they cause a specific disease known as trichinellosis (trichinosis). Human infection most commonly occurs from pork, and less frequently from wild game.

Fig. 28 Trichinella (Trichinella spiralis):

A — female and male; B — larvae encysted in pig muscle.

In the meat of pigs infected with Trichinella, one can find scattered, small, oval cyst-like bodies containing a tightly coiled spiral larva (a tiny worm) about 0.5 mm long. These are young "muscle trichinellae." If the meat is insufficiently sterilized—i.e., eaten undercooked or underboiled by a host (human, pig, rat, etc.)—the cysts dissolve in the host's Stomach, and the young trichinellae emerge, migrating to the small intestine. The number of trichinellae entering The Human Body can be enormous.

In the small intestine, the trichinellae grow and reach sexual maturity within 3 days. Following copulation, the fertilized females attach their anterior ends to the intestinal mucosa. Trichinellae are characterized by ovoviviparity. Over its lifetime (about 2 months), a single female produces up to 2,000 larvae. The young larvae penetrate the Lymphatic vessels and subsequently enter the Circulatory system, carried by the blood to all PARTS OF THE body. Next, the larvae actively invade the cross-striated muscle fibers, where they feed, grow, and ultimately destroy the Muscle tissue. In about 2 weeks, the larvae settle down, coil into a spiral, and gradually become enclosed in a Connective Tissue capsule. Approximately one year after cyst formation, CaCO₃ is deposited in its walls, giving the capsule a milk-white appearance.

Thus, the entire life cycle of Trichinella takes place inside the host's body without any stage spent in the external environment. Sexually mature worms develop in the host's intestine, during which time the host serves as the primary (definitive) host. However, once the parasite larva invades the musculature, that same individual organism becomes an intermediate host. Symptoms of trichinellosis begin with the penetration of females into the intestinal wall, manifesting as gastrointestinal disorders and facial edema. These are followed by chills, fever (sometimes reaching 40°С or higher), headache, body aches, and severe pain upon chewing, swallowing, or moving the eyes. The disease can last up to 1.5 months and, in cases of heavy infection, may prove fatal.

As a preventive measure, pork carcasses intended for sale are inspected by examining small pieces of muscle under a Microscope. Infected carcasses are destroyed. Proper sanitation must be maintained in pig farming, and pigs must be prevented from eating dead rats or slaughterhouse waste from other pigs.

Phylogeny of the Phylum Nemathelminthes (Roundworms)

Only recently, following a more in-depth Study of the groups belonging to this phylum, has it become clear that all of them share phylogenetic roots with Flatworms (turbellarians).

Indeed, individual organizational features common to flatworms are scattered across various classes of the phylum Nemathelminthes. For instance, primitive rotifers and gastrotrichs retain patches of the body covered with cilia. In these classes, as well as in spiny-headed worms (acanthocephalans), the excretory organs are represented by typical protonephridia. The nervous system of nematodes does not contradict the basic structural plan of flatworms.

Significant differences between roundworms and flatworms include the presence of a primary body cavity (pseudocoel) and a hindgut with an anus. However, these features are the result of progressive evolution and do not invalidate the aforementioned theory regarding the Water/144.html">Origin of the phylum Nemathelminthes. Interestingly, in some rhabdocoel turbellarians, the lacunae within the parenchyma are so large that parasitic Ciliates can swim freely within them. The enlargement and fusion of such lacunae could have led to The formation of an extensive primary body cavity.

The structure of individual classes within Nemathelminthes shows, at first glance, marked differences from turbellarians. Pseudocoelomate worms are characterized by heavy cuticularization, the more or less complete disappearance of the ciliated epidermis, and a fixed cell constancy (eutely), observed in rotifers and to some extent in nematodes. Rotifers have developed a specialized trochal (rotary) apparatus. The nervous system has undergone profound modifications, the pharynx has transformed into a masticatory organ, the skin-muscular sac has broken down into separate muscles, and so on.

All these differences pertain mainly to individual organs and Tissues, leaving the fundamental body plan largely unaffected.

Self-assessment and Review Questions:

1. What general features characterize roundworms?

2. What is the Internal Structure of roundworms?

3. What harm does the human roundworm (Ascaris lumbricoides) inflict on its host?

4. What environmental conditions are necessary for the completion of The life cycle of the human roundworm?

5. What preventive measures are required to avoid ascariasis?

6. How do the pinworm and the roundworm differ?

7. What preventive measures are necessary to prevent enterobiasis and trichinellosis?