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

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

PHYLUM COMB JELLIES (CTENOPHORA)

CLASS TREMATODES, OR DIGENETIC FLUKES (TREMATODA, OR DIGENEA)

All trematodes are endoparasites. Adults (maritae) are found primarily in various sections of the digestive tract of vertebrates, as well as in their Lungs, Kidneys, body cavity, and Circulatory system. Many of them are causative agents of severe diseases in humans and livestock. The size of trematodes ranges from 0.3–0.4 mm to 3–7.6 cm. Over 4,000 species are known, with about 600 species recorded in Ukraine.

The body shape of these animals is most commonly leaf-like. Adult worms, known as maritae, typically possess two well-developed suckers. One of them (the oral sucker) is located at the anterior end of the body and contains the Mouth opening in its center; the second (the ventral sucker) Functions solely as an attachment organ. It is usually located in the center of the body, although in some species it is shifted closer to the anterior or posterior end. The degree of sucker development depends on the site of parasite localization in the host.

In trematodes parasitizing the intestinal tract, the suckers are robust, whereas in those inhabiting the body cavity or bloodstream, they are underdeveloped or entirely absent.

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Fig. 143. Diagram of The Structure of trematode tegument (based on Electron Cell/15.html">Microscopy data):

1 — outer anuclear cytoplasmic layer; 2 — Mitochondria; 3 — basement membrane; 4 — cytoplasmic strands connecting the outer and sunken Regions of the epithelium; 5 — sunken cytoplasmic areas with nuclei; 7 — circular Muscles;

8 — longitudinal muscles; 9 — cuticular spine

Unlike free-living turbellarians, the integument of trematodes and all other parasitic worms lacks cilia and is represented by a tegument structured as a sunken epithelium (Fig. 143). The outer PARTS OF THE epithelial Cells fuse along their lateral surfaces to form a syncytium, which is a cytoplasmic layer devoid of cell membranes. Cytoplasmic processes containing nuclei extend from it and reach the parenchyma. The cytoplasmic layer is underlain by a basement membrane that is interrupted at the points where cytoplasmic processes branch off. Beneath the membrane lie circular and longitudinal Muscle layers.

The Digestive System of trematodes is generally well developed and similar to that of turbellarians, although some smaller forms exhibit its reduction. The digestive system begins with the mouth opening, which leads into a muscular Pharynx that passes into the Esophagus. The esophagus continues into two blindly ending Branches of the midgut.

In large trematodes, such as Fasciola hepatica, the branches of the midgut are branched—which is associated with their transport function—and all ramifications end blindly (Fig. 144).

Fig. 144. Digestive system of trematodes — Opisthorchis (a), Fasciola (б), Gymnophalus (в): 1 — mouth opening; 2 — pharynx; 3 — esophagus; 4 — ventral sucker; 5 — midgut

The excretory system of these animals is of the protonephridial type; the main collecting ducts empty into a Urinary Bladder that opens to the outside via an excretory pore.

The Nervous system of trematodes is a typical orthogona. It consists of an orthogonal Brain from which nerve trunks extend forward, innervating the oral sucker and branching into the outer cytoplasmic layer of the tegument at the anterior end of the body. Extending backward from the brain are two ventral trunks (the thickest ones), two dorsal trunks, and two lateral trunks. All longitudinal trunks are interconnected by transverse commissures forming rings or half-rings. Most of these are located in the anterior part of the body. Sense Organs in adult forms are poorly developed, which is associated with their parasitic lifestyle.

The Reproductive System of the marita is hermaphroditic. An exception is found in the dioecious Blood flukes of the family Schistosomatidae.

The Male Reproductive System is represented by two, less frequently several, Testes of various shapes, ranging from spherical to treelike-branched.

Vasa efferentia extend from the testes and merge into a common duct that begins with a dilation, the Seminal Vesicle. This vesicle passes into the ejaculatory duct, which penetrates the copulatory organ. It opens into the genital atrium and can be everted outwards (Fig. 145).

Fig. 145. Diagram of the reproductive system structure in a trematode marita:

1 — genital atrium; 2 — vitelline glands; 3 — vitelline ducts; 4 — Uterus; 5 — testes; 6 — urinary bladder; 7 — vasa deferentia; 8 — ootype; 9 — Mehlis' gland; 10 — seminal receptacle; 11 — Ovary; 12 — copulatory organ

The FEMALE REPRODUCTIVE SYSTEM consists of a single ovary of variable shape. A short oviduct leads to a small pouch-like structure known as the ootype. It receives the duct of the seminal receptacle, numerous unicellular glands that form Mehlis' gland, and the canal of a reservoir where the secretions of the paired vitelline glands accumulate. Originating from the ootype are Laurer's canal, which opens on the dorsal surface, and the uterus, shaped as a long, convoluted tube that opens into the genital cloaca. During copulation, the copulatory organ of one individual is inserted into the uterus of another; spermatozoa travel through it via the ootype into the seminal receptacle, where they are stored throughout life. Actual Fertilization of the eggs takes place in the ootype, into which eggs enter one by one from the ovary; spermatozoa in small portions, along with vitelline cells, also arrive here from the seminal receptacle. The fertilized egg becomes surrounded by vitelline cells and enclosed in a tough shell formed by the secretions of Mehlis' gland and the vitelline glands; the fully formed eggs pass into the uterus and move along it toward the excretory pore, gradually maturing. Unused spermatozoa and excess vitelline cells are expelled to the exterior through Laurer's canal. Trematodes are exceptionally prolific, producing on average a new egg every 20–30 seconds.

All trematodes exhibit a highly complex life cycle characterized by Morphology/12.html">ALTERNATION OF GENERATIONS, parasitic and free-living developmental phases, and a change of hosts (Fig. 146).

Fig. 146. GENERALIZED SCHEME OF the trematode life cycle: parasite in the definitive host (a), in the external environment (b), in the first intermediate host (c), In the second intermediate host (d)

As already mentioned, maritae parasitize the Internal Organs of vertebrate animals. Typically, the eggs produced by the parasites must reach Water, where they hatch into motile larvae known as miracidia (Fig. 147). The body of a miracidium is generally covered with large, flattened ciliated cells (epithelial plates), whose Cytoplasm contains substantial reserves of Glycogen serving as the energy source required for ciliary beating. Beneath the epithelial plates lie the hypodermis and layers of circular and longitudinal muscles. Miracidia lack a digestive system; they do not feed and survive entirely on stored nutrients. The excretory system is represented by two protonephridia with two to four cyrtocytes opening via independent pores. Miracidia possess a cerebral ganglion that innervates well-developed sense organs, including a pair of inverted eyes and sensilla of presumably diverse functions (chemoreceptors, mechanoreceptors, etc.).

At the anterior end of the body lies a proboscis, which plays a crucial role in the penetration of the miracidium into the body of the first intermediate host, typically a mollusk of a specific species, predominantly from the Class Gastropoda. The ducts of the apical gland open at the tip of the proboscis; its secretion breaks down the mollusk's integument, thereby facilitating miracidial entry. The apical gland is referred to as the penetration organ. Germ Cells are located in the posterior region of the body of the miracidium. Within a short time, miracidia must locate and penetrate the first intermediate host. During this period, Taxes play a major role in facilitating their encounter. For instance, the miracidia of many species exhibit negative geotaxis and positive phototaxis; the combination of these responses causes them to gather near the water surface, where pulmonate snails—the intermediate hosts of these trematodes—regularly ascend to breathe. Larval responses to chemical stimuli are also critically important. In many trematode species (Opisthorchis felineus, Dicrocoelium dendriticum), miracidia do not hatch into the external environment, and mollusk infection occurs when they ingest eggs containing fully formed larvae.

Fig. 147. Structure of a miracidium — miracidium of Fasciola hepatica (a), arrangement of epithelial plates (b): 1 — retracted proboscis; 2 — apical gland; 3 — cilia; 4 — sensilla; 5 — cerebral ganglion; 6 — cyrtocyte; 7 — protonephridial duct; 8 — bladder; 9 — germinal cells; 10 — muscle layer; 11 — developing embryos; 12 — pigmented eyespot; 13 — glandular cell; 14 — epithelial plates

Upon penetrating the mollusk's body, the miracidium transforms into a sexually mature mother sporocyst, which has a significantly simpler Organization than the miracidium. Its eyes, proboscis, apical gland, and other structures disappear. The larva sheds its epithelial plates, and the underlying hypodermis becomes the tegument of the sporocyst, representing a case of regressive metamorphosis. The body of the sporocyst is sac-like, rounded, elongated, or worm-like. In some trematode species, sporocysts are branched (Fig. 148). The sporocyst lacks a gut and absorbs nutrients across its entire body surface. Its nervous system and sense organs are poorly developed.

Fig. 148. Sac-like (a) and filamentous (b) sporocysts:

1 — germ balls; 2 — cercariae; 3 — rediae

The mother sporocyst reproduces parthenogenetically. Parthenogenetic eggs, which fill the body of the sporocyst, begin to cleave without fertilization, forming embryos known as germ balls, from which the second parthenogenetic generation of trematodes develops—represented by either rediae or daughter sporocysts. Typical rediae possess an elongated cylindrical body with a pair of locomotor appendages located in the posterior third of the body. The tegumentary-muscular sac is well-developed in rediae, and they feature a digestive system starting with a mouth opening that leads to a pharynx and further into a long, sac-like gut. The protonephridia are paired, each with its own excretory pore; the nervous system is of the orthogon type, though sense organs are poorly developed (Fig. 149).

Daughter sporocysts are considered by scientists to be neotenic rediae, meaning rediae that begin reproduction at Cytology/cytology/16.html">Early stages of organismic development. Daughter sporocysts are sac-like, worm-like, or rounded in shape and lack a digestive system. Unlike the mother sporocyst, a daughter sporocyst possesses a birth pore (similar to rediae) through which the next generation of trematodes—the cercariae—emerges.

The release of rediae and daughter sporocysts, which are quite active at a young age, from the mother sporocyst occurs upon their simultaneous maturation through the rupture of its walls or via the gradual formation of individuals through small ruptures that quickly heal, allowing the sporocyst to continue living and reproducing.

Fig. 149. Structure of rediae and cercariae — typical rediae (a, b), sac-like redia (c), cercariae (d, e): 1 — pharynx; 2 — intestine; 3 — locomotor appendages; 4 — vestigial intestine; 5 — cercarial embryos; 6 — fully formed cercariae; 7 — tail appendage

Rediae and daughter sporocysts localize within the body cavity of the mollusk, attaching to The surface of its intestine and other internal organs, but most frequently they parasitize the digestive gland (Liver). Rediae actively feed on liver Tissues and can additionally absorb nutrients across their entire body surface. Sporocysts feed solely through their body covering. The tegument of the sporocyst is covered with numerous microvilli, and specialized glandular cells secrete all the necessary digestive Enzymes that break down adjacent host tissue adhering to the sporocyst wall; thus, membrane Digestion occurs, with the difference that it takes place outside the Organism rather than inside.

Reproduction of rediae and daughter sporocysts proceeds parthenogenetically from germ cells which, similarly to the mother sporocyst, develop within their body cavity. Daughter rediae and sporocysts, or directly cercariae, can develop from these germ cells. The productivity of rediae is relatively low (two to three daughter rediae or six to ten fully formed cercariae), whereas a single sporocyst can produce hundreds of thousands of offspring. This is due to polyembryony, namely The Development of several and sometimes many embryos from a single egg cell.

Fully developed cercariae closely resemble maritae in structure, differing in their smaller size and the presence of a muscular tail of variable morphology (see Fig. 149). They possess suckers, a nearly complete digestive system (which, however, remains non-functional), a protonephridial excretory system, and a well-developed nervous system. Sense organs are represented by numerous sensilla sensitive to chemical and mechanical stimuli. Cercariae possess various glands whose degree of development depends on their biology. For instance, in larvae that encyst in the external environment, cystogenous glands are most developed; their secretion is expended on forming the thick cyst wall. In cercariae that undergo further parasitism in a second intermediate host, these glands are less developed. The penetration gland complex is highly developed, with secretions aiding in piercing the host's integument. Additionally, mucous glands are present; their secretion coats the cercaria's body in a sheath, protecting it from the mollusk's enzymes as the cercaria migrates through its tissues.

Cercariae of certain species do not require a second intermediate host. Some of these settle on a suitable substrate and encyst there, transforming into adolescent stages known as adolecariae. Covered by a thick two- to four-layered wall, the nearly motionless adolescent larva very slowly expends its glycogen reserves and thus remains viable for a prolonged period (though without growing or developing) until ingested by the definitive host. Other cercariae actively penetrate the body of the definitive host.

In many species, further development occurs within a second intermediate host (fish, aquatic insect larvae, other Mollusks, crustaceans, etc.). Within 24–48 hours (the time required to exhaust the glycogen reserves stored in the larval body, particularly in the tail appendage), cercariae must encounter a second intermediate host. Much like miracidia, cercariae possess a series of adaptations (geo-, photo-, and chemotaxes) that facilitate host Location and penetration.

Penetration through the integument of a new host varies among species. Some cercariae, for example, feature a strong cuticular stylet used to cut the integument, whereas others possess a short, robust proboscis armed with hooks that damage the surface while penetration gland secretions are injected into the wound. The penetration site is identified using sensilla concentrated at the anterior end of the cercaria's body. Upon entering the host, cercariae cast off their tails. In the second intermediate host, they localize in muscles or internal organs, where they encyst and transform into metacercariae. Alongside active penetration, passive entry into the second intermediate host also occurs. For instance, larvae of certain trematodes may be swallowed by fish or drawn in with water and encyst on the walls of the Oral Cavity. There are species whose cercariae settle on the snail's body surface and subsequently crawl into the pulmonary cavity to encyst. The cysts possess a thin protective wall through which nutrients can diffuse. The enclosed larva (metacercaria) grows and develops, but—just like the adolescent form—metacercariae transform into maritae exclusively within the definitive host.

Thus, the trematode life cycle alternates between a single hermaphroditic generation (marita) and several parthenogenetic generations (sporocysts, rediae). This type of cycle is termed heterogony. The adult of the hermaphroditic generation is the marita, while those of the parthenogenetic generations are the sporocyst and redia. The miracidium is the larva of the sporocyst, whereas the cercaria, metacercaria, and adolecaria are larvae of the marita.

As previously mentioned, trematodes include A large number of species responsible for dangerous diseases (trematodiases) in humans, domestic animals, and livestock. The pathogenic impact of trematodes on the host organism is multifaceted. The parasites disrupt tissue integrity, cause blockage of various organ ducts, and negatively affect the host through The excretion of toxic Metabolic waste products. In cases of heavy infections (infestations), host exhaustion is observed. Furthermore, trematodes can induce the development of various neoplasms, including malignant ones.

The Classification of the Class Trematoda is currently undergoing revision. Scientists distinguish anywhere from four to ten orders within this class. Let us examine the life cycles of the most prominent trematodiases agents.

Fig. 150. Life Cycle of Fasciola hepatica: marita (a), fertilized egg (b), miracidium (c), young mother sporocyst (d), mature sporocyst (e), redia (f), cercaria (g), adolescent metacercaria/adolescaria on grass (h), definitive hosts (i), intermediate host — the mollusk Lymnaea truncatula (j). 1 — oral sucker; 2 — genital atrium; 3 — copulatory canal; 4 — ejaculatory duct; 5 — Mehlis' gland; 7 — vitelline ducts; 8 — vitellarium; 9 — sperm duct; 10 — Testis; 11 — oviduct; 12 — ovary; 13 — ventral sucker; 14 — intestine; 15 — pharynx; 17 — penetration gland; 19 — germ cells; 20 — germ balls; 21 — mother redia; 22 — daughter redia; 23 — tail

The liver fluke (Fasciola hepatica) is a fairly large (26–3 cm long) leaf-shaped parasite dwelling in the Bile ducts of the liver in sheep, cattle, and less frequently in other animals and humans. The oral and ventral suckers are closely positioned, and both the bifurcated intestine and the reproductive glands are highly branched (Fig. 150).

The maritas of F. hepatica are exceptionally prolific; a single individual produces about a million eggs per week, which are expelled outward through the bile ducts and intestine. The development of eggs and the hatching of miracidia occur exclusively in water. The first intermediate host is the truncatulate pond snail (Galba truncatula). Upon penetrating the mollusk's body, the miracidium transforms into a mother sporocyst, which generates a new generation—rediae. These burst through the mother sporocyst, enter the mollusk's body cavity, and migrate to the digestive gland. Here, several generations of rediae are produced, followed by cercariae that emerge into the water, swim briefly, attach themselves to aquatic plants or the water surface film using their suckers, discard their tails, and encyst to become metacercariae (adolescariae). The encysted adolescariae withstand desiccation and retain viability even in dry hay. Consequently, humans can become infected by drinking water from stagnant water bodies, while animals may also contract the parasite by consuming plants in marshy pastures or hay.

Fascioliasis is a globally distributed disease, although it occurs quite rarely in humans.

The feline liver fluke, Opisthorchis felineus, is the CAUSATIVE AGENT OF a highly dangerous disease known as opisthorchiasis (Fig. 151). The parasite localizes in the bile ducts of the liver, the Gallbladder, and occasionally in the pancreatic ducts of humans and a very broad range of mammals. The first intermediate host is the freshwater snail Bithynia leachi, and the second comprises various species of cyprinid fish. The cercariae penetrate the integument of fish and encyst in their muscles, transforming into metacercariae. Infection occurs through the consumption of inadequately salted, boiled, or fried fish. However, opisthorchiasis is most prevalent in regions where frozen fish, known as 'strogonina', is consumed. The intensity of infection can be extremely high. During the autopsy of a human who died from opisthorchiasis, 25,320 trematodes were discovered in their liver. Opisthorchiasis is a widespread disease, notably in Ukraine.

Fig. 151. Life cycle of Opisthorchis felineus: definitive hosts (a), marita (b), egg with miracidium (c), first intermediate host — the mollusk Bithynia leachi (d), sporocyst (e), redia (f), cercaria (g), second intermediate host (h), metacercaria (i): 1 — oral sucker; 3 — pharynx; 3 — intestine; 4 — copulatory organ; 5 — ventral sucker; 6 — vitellaria; 7 — uterus; 8 — vitelline ducts; 9 — seminal receptacle; 10 — ovary; 11 — testes; 12 — excretory bladder

The lancet liver fluke (Dicrocoelium dendriticum) is the causative agent of dicrocoeliasis. It parasitizes the hepatic bile ducts of sheep, cattle, occasionally humans, and many wild animals (Fig. 152). The development of this fluke is not tied to an aquatic environment. Dicrocoelium eggs containing miracidia reach the soil along with animal excrement. They are covered by very thick shells, allowing them to resist drying out for several months. The primary intermediate host of the parasite is terrestrial snails (such as Helicella, Zebrina, etc.), which ingest the eggs. Inside the snail's intestine, the miracidium hatches from the egg, penetrates the liver, and transforms into a mother sporocyst, within which

daughter sporocysts develop, which in turn directly give rise to cercariae (the redia stage is bypassed). The cercariae migrate into the pulmonary cavity of the snail, where they become coated in mucus, clump together, and are expelled through the snail's respiratory pore in the form of 'slime balls' (collective cysts) containing thousands of cercariae.

Fig. 152. Life cycle of Dicrocoelium dendriticum: marita (a), egg with miracidium (b), sporocyst (c), cercariae in a mucous slime ball (d), metacercariae in the brain and body of an ant (e), second intermediate host — ant (f), definitive hosts (g), first intermediate host — terrestrial snail (h): 1 — oral sucker; 2 — pharynx; 3 — intestine; 4 — ventral sucker; 5 — testes; 6 — vitellaria; 7 — uterus; 8 — ovary; 9 — ejaculatory duct; 10 — copulatory organ; 11 — oviduct

The second intermediate host of the lancet liver fluke is ants of the genus Formica. They transport the slime balls to their anthills and consume them, thus becoming infected with Dicrocoelium. Mass infection of the ants is facilitated by their habit of sharing food with other colony members. In an ant that has eaten a slime ball, one of the cercariae migrates to the brain,

transforming into a 'brainworm', while all the others migrate to the musculature, where they turn into metacercariae. Under the Influence of the 'brainworm', infected ants experience paralysis; they crawl up onto grass stems, clamp their mandibles around the edge of a leaf blade, and hang motionlessly. Herbivorous animals ingest them along with the grass. In the digestive tract of these animals, the ants are digested, and the metacercariae are released from their cysts, migrate into the liver bile ducts and gallbladder, and there develop into maritas. Humans can become infected by accidentally swallowing an infected ant, but this occurs very rarely.

Specific parasites of humans include trematodes of the genus Schistosoma — Sch. haematobium and Sch. mansoni, which parasitize the host's bloodstream. A distinctive feature of these trematodes is dioecism (separate sexes). The male, being broader than the female, embraces her with his body edges folded toward the ventral side (Fig. 153). The life cycle of schistosomes involves only two hosts. The maritas localize in the mesenteric Veins (the causative agents of intestinal Schistosomiasis, Sch. mansoni) or in the venous plexuses of the human Urogenital System (Sch. haematobium). Schistosome eggs settle in capillaries and penetrate deep into tissue layers. Here, through muscular contractions, they make their way into the intestine or urinary bladder and are expelled outward.

Fig. 153. Life cycle of Schistosoma haematobium:

a — mature male and female; b — miracidium; c — sporocyst; d — redia; e — cercaria; f — cercaria in waiting posture; g — definitive host; h — intermediate host

For further development, the eggs must reach water, where the miracidium seeks out a specific species of snail, inside which all subsequent transformations take place, including the Development of the sporocyst and The formation of rediae and cercariae. Cercariae exhibit positive phototaxis, which causes them to float up to the water surface film and remain motionless in a 'waiting posture' for several days. Upon the appearance of a human, the cercariae rapidly attach to them. Infection occurs through active penetration of human Skin during swimming, working in rice paddies, and similar activities. Within The Human Body, schistosomes can live for several years, causing quite severe Disorders of the urogenital organs.

Human schistosomiasis is a disease of southern and tropical countries. It does not occur in Ukraine; however, individuals who swim in stagnant, overgrown ponds or river bays may experience schistosome dermatitis, a skin condition known as swimmer's itch. This disease is caused by cercariae of schistosome species that parasitize waterfowl, such as ducks and gulls. The irritation of human skin is associated with the mechanical action of the larvae burrowing into the epidermis and the chemical substances they secrete. The cercariae perish within human skin. Dermatitis occurs quite frequently in Ukraine, Russia (Volga Delta), certain regions of Kazakhstan, and the United States.

In Conclusion, mention should be made of the life cycle of flukes from the genus Leucochloridium, which parasitize birds. Eggs containing fully formed miracidium larvae are dispersed by birds via excrement and are subsequently ingested by terrestrial amber snails (Succinea). Miracidia hatch within their intestine, migrate to the liver, and transform into sporocysts, inside which tailless cercariae develop. The sporocyst grows and branches, and its elongated terminal branches—engorged with developing cercariae—invade the snail's tentacles, causing them to swell significantly. Furthermore, these branches of the sporocyst acquire a bright coloration (green, brown, frequently marked with white stripes and reddish-brown spots). The coloration and pulsing Movements of the sporocyst are clearly visible through the thin integument of the tentacles, making them highly attractive targets for foraging birds (Fig. 154).

Fig. 154. Leucochloridium paradoxum: marita (a), infected amber snail Succinea (b), sporocyst from the snail's liver (c): 1 — distended snail tentacles with sporocyst outgrowths; 2 — pigmented sac-like outgrowths of the sporocyst