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

Multicellular Animals
Flatworms
Class Trematoda

The Class of flukes (Trematoda) comprises about 4,900 species of worms that inhabit the Internal Organs of invertebrates and vertebrates, and occasionally humans. Their length ranges from a few millimeters to 8-9 cm, and their body shape is typically leaf-like. They usually possess two suckers: one surrounding the Mouth and the other located on the ventral side of the body, which serve to anchor the parasite within the host's Organism. The body of flukes is covered by a complex specialized outer layer (tegument) that prevents the parasite from being digested by the host's digestive juices. Beneath this covering lie layers of circular, oblique, and longitudinal musculature. Flukes are relatively sluggish.

The Digestive System includes a mouth positioned in the middle of the oral sucker, a prepharyngeal cavity, a muscular Pharynx (these sections form the sucking apparatus), and a blindly ending, branching midgut. The excretory system consists of numerous branching canals that empty into an unpaired main channel, which opens at the posterior end of the body via an excretory pore (Fig. 15).

The Nervous system is of the ladder-type (or orthogon). It includes a pair of nerve nodes (ganglia) located on the sides of the pharynx. Nerves extend from them to the oral sucker, while three pairs of longitudinal nerve trunks extend posteriorly (the most developed being the ventral, lateral, and dorsal trunks), which are interconnected by transverse nerve commissures. Sense Organs are extremely poorly developed. Respiration takes place in an oxygen-free environment through a Fermentation-like process that produces acids toxic to the host, such as valeric, butyric, and succinic acids, among others.

Fig. 15 Anatomy of a fluke:

A — nervous system: 1 — circumpharyngeal nerve ring; 2, 3 — longitudinal nerve trunks; B — excretory system: 1 — numerous excretory ductules; 2 — main excretory trunk; 3 — excretory pore.

Flukes are hermaphroditic and exhibit a highly complex life cycle involving alternation of hosts and generations. In a typical scenario, it proceeds as follows. A hermaphroditic, sexually mature fluke (marita) parasitizes the intestines or other organs of vertebrate animals. The eggs it lays (which feature an operculum) are expelled from the host's body, most commonly with feces. For further development, the eggs of most species must reach Water.

In water, a ciliated larva called a miracidium hatches from the egg. In the posterior part of the miracidium's body lie specialized Germ Cells, or parthenogenetic eggs. The miracidium does not feed (surviving on Glycogen reserves) and swims in the water for a short time. To continue its development, it must enter the body of a gastropod mollusk, which serves as the intermediate host. Upon penetrating the intermediate host, the miracidium sheds its cilia and transforms into a sporocyst—a stage capable of reproduction. The parthenogenetic eggs enclosed within the sporocyst undergo Cleavage, giving rise to embryos of the next daughter generation, known as rediae. These differ from the sporocyst by being motile and eventually emerging from it.

Next, in the same manner that rediae were formed within the sporocyst, a new generation of larvae—cercariae—develops from germ cells inside the rediae. These already resemble adult flukes. A distinguishing feature is the presence of a long, muscular, and mobile tail with a sharp spine (stylet) at the posterior end of the body. Cercariae emerge from the redia and subsequently leave the mollusk into the external environment. The further fate of these larvae varies. The cercariae of most trematode species must enter the body of a second intermediate (supplementary) host, which may include aquatic insect larvae, various species of Mollusks, fish, tadpoles, etc.

Using the stylet, cercariae penetrate the body of the secondary host, cast off their tail and stylet, settle in the host's internal organs, and encyst by secreting a thin, transparent envelope. This infective stage is called a metacercaria. Its further Development and transformation into a sexually mature individual is possible only if the secondary host is consumed by a larger vertebrate animal. In the intestines of these animals, the metacercariae break free from their cysts and complete their development.

Consequently, different Stages of the trematode life cycle occur in different hosts. The vertebrate animal in which the hermaphroditic generation of flukes parasitizes and reproduces sexually is called the definitive host. Conversely, animals in the bodies of which other generations and Developmental Stages of trematodes parasitize are called intermediate hosts. There may be one, two (most commonly), or more of them. Furthermore, the first intermediate host for trematodes is always some species of freshwater or terrestrial gastropod mollusk. The Role of the second intermediate host is played by various animals, but always those consumed by the definitive (primary) host. The latter becomes infected with trematodes by ingesting infective larvae with its food.

Various deviations from this outlined typical trematode life cycle are possible. Let us examine them using the life cycles of certain trematode species that quite frequently parasitize domestic animals and humans.

The Liver fluke (Fasciola hepatica) parasitizes the Bile ducts of the liver in sheep, cattle, and occasionally humans. The body is strongly flattened dorsoventrally, leaf-like, and measures 1.5-9.0 cm in length. The anterior part of the body is broad, tapering sharply toward the oral sucker, while the posterior part narrows gradually. The life cycle of the liver fluke involves only a single intermediate host.

Fig. 16 Development of the liver fluke (Fasciola hepatica):

A — miracidium: 1 — rudimentary intestine; 2, 3 — PARTS OF THE excretory system; 4 — germ cells; B, C — sporocysts with developing rediae; D, E — rediae with developing larvae; F — cercaria: 1 — oral sucker; 2 — ventral sucker; 3 — intestine; 4 — part of the Reproductive System; 5 — tail; G — adolescaria; H — young fluke.

The fertilized eggs of the liver fluke enter the bile ducts, pass into the host's intestine, and are subsequently expelled from the body with feces. For further development, the eggs must reach water, where microscopic larvae—miracidiums—hatch from them (operculate fluke eggs). These contain germ cells capable of developing without Fertilization (Fig. 16). To continue the life cycle, the larva finds an intermediate host, the freshwater gastropod mollusk known as the truncatulate pond snail (Limnaea truncatula), penetrates it, and transforms into a shapeless, motionless sac called a sporocyst, from the germ cells of which the next generation of larvae—rediae—emerges.

The Development of multiple larval generations from unfertilized egg cells leads to a dramatic increase in their numbers. As a result, far more larvae emerge from the intermediate host than the number of miracidiums that originally entered it. The larvae leaving the snail (cercariae) swim freely in the water for a time, then settle on aquatic plants, become encysted, and transform into adolescariae, which serve as the infective (invasive) stage in the liver fluke's life cycle. If an adolescaria is swallowed by an animal or occasionally a human, the cyst wall dissolves in the host's intestine, and the parasite migrates to the liver, where it reaches sexual maturity.

Thus, the Life Cycle of the liver fluke, like that of other members of the class, is characterized by such features as alternation of hosts (the sexually mature form lives in the body of a single definitive host, while the larva parasitizes the organism of an intermediate host) and The ability to reproduce asexually during larval stages. A hallmark of the liver fluke, shared with other parasitic worms, is The production of an enormous number of eggs. Although the alternation of hosts ensures the dispersal of the parasite with the "assistance" of intermediate hosts, it is inevitably associated with the mortality of numerous individuals at various stages of the life cycle (by no means all eggs reach water, not all miracidiums find intermediate hosts, etc.). In this context, The Significance of immense fecundity and larval multiplication becomes clear.

The lancet liver fluke (Dicrocoelium lanceatum) is a small (0.5-1.2 cm) worm that parasitizes primarily The Liver and bile ducts of small and large ruminants (Fig. 17). The first intermediate host of this fluke can be one of several species of terrestrial gastropod mollusks (such as the garden snail or slugs). The second intermediate (supplementary) host is an ant. Ants become infected by consuming clusters of cercariae enclosed in a mucous sheath, which are excreted by the mollusks. Infective larvae—metacercariae—develop within the bodies of infected ants. Herbivorous mammals become infected by ingesting infected ants along with grass.

Fig. 17 Lancet liver fluke (Dicrocoelium lanceatum):

1 — oral sucker; 2 — ventral sucker; 3 — pharynx; 4 — intestinal branches; 5 — Ovary; 6 — Uterus; 7 — seminal receptacle; 8 — vitelline glands; 9 — Testes; 10 — ejaculatory duct.

Prostogonimus (Prostogonimus ovatus) has a more oval (pear-shaped) form, reaching a size of up to 0.5-0.6 cm, and parasitizes the oviducts of birds. The disease it causes is characterized by the laying of soft-shelled eggs, as the eggs laid by infected birds lack a calcareous shell. The life cycle of Prostogonimus involves two intermediate hosts: the first is a small or large pond snail, and the second is a dragonfly larva (naiad). Birds become infected by consuming either the larvae or adult dragonflies, in whose bodies the infective metacercariae have formed.

The cat liver fluke (Siberian fluke) (Opisthorchis felineus) parasitizes the liver of dogs, cats, and humans. The parasite reaches a length of 0.8-1.3 cm and a width of 1.2-2 mm. The first intermediate host is the freshwater gastropod mollusk Bithynia leachi, while the second intermediate host comprises fish species such as roach, ide, and several others. Human infection, as well as infection in other fish-eating mammals, occurs through the consumption of raw or sun-dried fish containing encysted metacercariae of the cat liver fluke. This fluke is prevalent in Siberia and in the eastern and southern regions of Russia.

Unlike other flukes, the Blood fluke (Schistosoma haematobium) is dioecious. The longer (up to 20 mm) yet slender female resides in a deep ventral canal on the male's abdomen. These parasites always live in pairs. In this species, the eggs lack an operculum but feature a spine either on the side or at the posterior end. Schistosomes inhabit the large venous Vessels of the human Abdominal cavity, as well as the Veins OF THE Kidneys and Urinary Bladder. The eggs are excreted from the body in the urine. Upon reaching water, miracidia hatch from the eggs, much like in other flukes. Various species of freshwater gastropod mollusks serve as intermediate hosts.

Cercariae emerging from the snails enter the human bloodstream by actively penetrating the Skin during swimming or working in rice paddies. This results in a highly dangerous and severe disease known as Schistosomiasis. Damage to the Urinary System leads to nephritis, ulcers, and Hematuria. Pathological tissue overgrowths that can lead to malignant tumors are also possible.

There is a vast number of trematodes pathogenic to various domestic and commercial animals, as well as humans. Disease Prevention serves as a powerful tool in combating trematodes, relying heavily on an understanding of the biological patterns of parasite development.