INVERTEBRATE ZOOLOGY IN THREE VOLUMES - BOOK 1 - H.I. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

PHYLUM ROUNDWORMS (NEMATHELMINTHES)

CLASS NEMATODES (NEMATODA)

Subclass Enoplia

Most nematode species of this subclass inhabit the bottom of seas, freshwater bodies, and soils. Among them are commensals as well as animal and plant parasites.

The most characteristic feature of enoplians is the Progressive development of Sense Organs. The cephalic setae are arranged in two (6+10) or, more rarely, three (6+6+4) circles. Pocket-like amphids are well developed, numerous somatic setae are present, and some marine forms even possess eyes, though these can only distinguish the degree of illumination.

In many species, the stoma is armed with sharp onchia and Teeth.

The most important orders of the subclass are Enoplida, Dorilaimida, Mononchida, Mermithida, and Trichocephalida. Among them, enoplians (order Enoplida) are predominantly marine, free-living nematodes (predators and saprophages) with well-developed sensory setae on the anterior end of the body (Fig. 198, a).

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Fig. 198. Anterior ends of enoplians — sensory setae in marine enoplians (a), oral armature in the plant-parasitic dorilaimids Hoplolaimus tylenchiformis (b) and Dorylaimus striatus (c): 1 — sensory setae; 2 — teeth; 3 — stylet; 4 — spear

Dorilaimids (order Dorilaimida) are predators or plant parasites. Their Oral Cavity is armed with a robust spear used to pierce prey or Plant Cell Walls (Fig. 198, b — c).

Representatives of the order Mononchida are free-living soil and freshwater forms. Most of them are predators armed with onchia, the best-known being the single-toothed Mononchus papillatus (Fig. 199). Researchers consider this nematode promising for biological control against parasitic nematodes of valuable agricultural crops.

Fig. 199. Order Mononchida: Mononchus papillatus: 1 — papillae; 2 — onch; 3 — oral cavity

The order Mermithida includes parasites of freshwater and soil invertebrates, predominantly Arthropods (Fig. 200). Their larvae parasitize the host's body cavity, feeding through the cuticular integument. The final-stage larvae emerge from the host into Water or soil and transform into free-living adult mermithids. Mermithids are characterized by poorly developed sense organs and a peculiar midgut Structure lacking an intestinal lumen (the intestine appears as a solid cuticular tube surrounded by rows of nutrient-filled Cells).

Fig. 200. Locust nymph with the mermithid Agamermis decaudata (order Mermithida)

Mermithids attract researchers' attention as potential biological control agents against harmful insects, especially Blood-sucking dipterans.

Of the greatest practical importance are representatives of the order Trichocephalida.

The whipworm (Trichocephalus trichiurus) inhabits the cecum, and much less frequently, the human colon. It is a small (4–5 cm) worm whose anterior end is drawn out into a long Hair-like appendage, with which it deeply embeds itself into the intestinal mucosa and feeds on blood. The short posterior end is thickened and contains the intestine and Reproductive System (Fig. 201).

Fig. 201. Order Trichocephalida (Trichocephalus trichiuris):

1 — broadened posterior end; 2 — thread-like anterior end of the body embedded in the intestinal mucosa

Fertilized females lay eggs that are passed into the external environment with human feces. Humans become infected by swallowing the eggs through raw water, unwashed vegetables, and fruit. In the intestine, larvae equipped with a stylet hatch from the eggs. They penetrate the intestinal villi, where they remain for about ten days. Afterwards, the larvae emerge into the intestinal lumen and reach the cecum, attaching by their anterior end to its surface. During development, the larva molts four times; parasites reach sexual maturity in a month, and their lifespan is about five years. In cases of severe infection, the functioning of the digestive tract is disrupted, accompanied by nausea, pain, and reduced acidity. The destruction of the mucous membrane facilitates The entry of secondary bacterial infections into the body.

The trichina worm (Trichinella spiralis) is one of the most dangerous parasites affecting humans and many animal species (Fig. 202).

Fig. 202. Life Cycle of Trichinella spiralis (order Trichocephalida): adult female (a), male (b), larvae (c) and encysted larvae in Muscles (e), Circulation of the parasite in the host Organism (d), host range (e), pathways of Trichinella circulation under synanthropic conditions (є): 1 — meat scraps containing Trichinella larvae; 2 — circulation of Trichinella among rats

This is a small nematode (1.5–2 mm long). The developmental cycle of Trichinella is quite unique: its definitive and intermediate host is the very same individual. The parasites first reach sexual maturity in the animal's intestine (intestinal stage), and then their larvae encyst in its muscles (muscular stage). Thus, at no stage of development does the parasite emerge into the external environment.

The hosts of Trichinella include various mammals — carnivores, insectivores,

rodents, pinnipeds, and humans. Infection of a new host, including a human, occurs upon consuming a previous host that contains encysted Trichinella larvae. In the host's intestine, the larvae break out of their capsules and reach sexual maturity within three days. Trichinella are viviparous animals; while still in the female's Uterus, larvae hatch from the eggs and are born through the genital opening. Throughout its lifetime (about 50 days), a female produces up to 2,000 larvae. Passing through the host's intestinal wall, the larvae enter the Lymphatic vessels, and from there into the Circulatory system. They are distributed throughout the body, but ultimately settle in Cytology/practical/58.html">Striated Skeletal Muscle. There, the larvae penetrate the muscle fibrils, destroy them, and on the 17th–18th day coil into a spiral shape (hence their species name). Over the course of two to three months, a capsule made of host Connective Tissue forms around them.

The walls of the capsule become permeated with Blood Vessels and nerve endings. Along with the host's blood, the parasite receives the necessary nutrients and oxygen, and the larval Metabolic waste products are also excreted via the blood. Beginning in the sixth month, the capsule walls become impregnated with calcium carbonate, yet the larvae remain viable for a long time. The muscles most frequently affected are those of the Tongue, forearm, Diaphragm, masseter muscles, intercostal muscles, and calves, among others. This process is extremely painful for humans or other hosts — patients experience fever (up to 40°C and higher), facial Swelling, altered blood composition, and muscle pain, especially during chewing, swallowing, and Eye Movements. The acute phase of the disease can last for a month and a half and, in cases of severe infestation, lead to death.

In nature, trichinellosis is transmitted when animals prey on one another (rodents, small carnivores, badgers, foxes, wolves, bears, etc.). Humans become infected by eating poorly cooked or fried pork, pigs become infected by eating rats or slaughterhouse waste, and rats by eating individuals that have died of trichinellosis. Humans can also contract the disease through the meat of wild boars, bears, and other game.

Completely curing trichinellosis remains a challenge. Preventive measures are of decisive importance: mandatory microscopic examination of pork intended for sale, discarding carcasses infected with Trichinella, establishing sanitary conditions that rule out the possibility of domestic animal infection, and rat eradication. Trichinellosis is classified as a natural focal disease. Such foci also exist in certain regions of Ukraine, Belarus, and Russia. Recently, it has been established that T. spiralis is a aggregate species comprising four distinct species.