INVERTEBRATE ZOOLOGY - G. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE METAZOA (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERALLY SYMMETRICAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM FLATWORMS (PLATHELMINTHES)

SUBPHYLUM NEODERMATA (NEODERMATA)

SUPERCLASS ACERCOMERA (ACERCOMERA)

CLASS TREMATODES (TREMATODA)

One of the most numerous classes of Flatworms, comprising over 4,000 species (with more than 600 in Ukraine). All trematodes are endoparasites. Sexually mature hermaphroditic individuals (maritae) parasitize vertebrates and humans, locating in various sections of the Digestive System, Lungs, Kidneys, body cavity, Circulatory system, etc. Many of them are causative agents of severe diseases in humans and agricultural animals.

Trematodes are characterized by a specific tegument Structure; attachment Organs—oral and ventral suckers; as well as a complex life cycle—heterogony—featuring the alternation of sexual and parthenogenetic reproduction with changes of hosts.

Morphology. The body of trematodes is leaf-shaped, less frequently elongated or thickened. Sizes range from 0.3 to 76 mm (giant Liver fluke - Fasciola gigantica). The marita possesses two suckers: the oral sucker, located at the anterior end of the body, and the ventral sucker, which Functions exclusively as an attachment organ and may vary in position among different species. The suckers are equipped with powerful circular and radial Muscles.

Skin-muscular sac. The integument is represented by a tegument built as a sunken epithelium, consisting of a surface, non-ciliated continuous layer of Cytoplasm without Cell boundaries (syncytium) covered by a cytoplasmic membrane and a glycocalyx layer, and epithelial cell bodies embedded in the parenchyma (Fig. 121). The tegument performs a protective function, and through it, nutrients are also absorbed from the host's body. Beneath the tegument lie circular and longitudinal Muscle layers.

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Fig. 121. Schematic diagram of trematode integument structure (based on Electron Microscopy data) (from Dogiel):

1 - outer anuclear cytoplasmic plate;

2 - Mitochondria; 3 - basement membrane; 4 - cytoplasmic strands connecting the outer and sunken PARTS OF THE epithelium;

5 - sunken regions of cytoplasm with nuclei; 6 - nuclei; 7 - circular muscles; 8 - longitudinal muscles; 9 - cuticular spine

The parenchyma is well developed, filling all the spaces between Internal Organs.

Digestive system. The Mouth opening is located at the bottom of the oral sucker. The foregut is represented by a muscular Pharynx and Esophagus. The midgut is bifurcated (Fig. 122), sometimes forming a ring. In larger forms, the Branches of the midgut branch out and possess numerous blind caeca, which facilitates The transport of nutrients to various organs; in smaller forms, the intestine is often rudimentary or sometimes entirely absent.

Fig. 122. Types of the digestive system in trematodes (from Ginetsinskaya): a - Opisthorchis; b - Fasciola; c - Gymnophalus:

1 - oral sucker; 2 - pharynx; 3 - esophagus; 4 - ventral sucker; 5 - midgut

Maritae feed on intestinal contents, Tissues, and Blood of the host, and additionally, are capable of absorbing dissolved organic substances (Monosaccharides, Amino Acids) directly through the integument.

Excretory system is of the protonephridial type. Numerous tubules from cyrtocytes empty into two main canals; at the posterior end of the body, they unite into a bladder that opens outward via an excretory pore.

The Nervous system is orthogon-type, consisting of a cerebral ganglion with nerves extending to the oral sucker, and several longitudinal nerve trunks running to the posterior end of the body, interconnected by numerous commissures.

Sense Organs are poorly developed due to the parasitic lifestyle. However, the free-swimming larva, the miracidium, possesses inverted eyes and numerous epidermal sensilla.

The Reproductive System of flukes is hermaphroditic (Fig. 123) (with the exception of blood flukes, which are gonochoristic). The Male Reproductive System is represented by two Testes from which two vas deferens depart. The latter unite into an ejaculatory duct that penetrates the copulatory organ, the cirrus, and terminates in the genital cloaca.

Fig. 123. Diagram of the trematode reproductive system (after Dogel):

1 - genital cloaca; 2 - vitellaria; 3 - vitelline ducts; 4 - Uterus; 5 - testes; 6 - Urinary Bladder;

7 - vas deferens; 8 - ootype; 9 - Mehlis' gland; 10 - seminal receptacle; 11 - Ovary; 12 - copulatory organ

The FEMALE REPRODUCTIVE SYSTEM includes a single ovary with an oviduct leading from it. The oviduct empties into a small reservoir, the ootype, which also receives ducts from the vitellaria and seminal receptacle, as well as numerous glands forming Mehlis' gland. A short Laurer's canal and the uterus, which typically appears as a convoluted tube, extend from the ootype. The uterus opens into the genital cloaca. Fertilization of the oocytes occurs in the ootype, after which each egg is surrounded by several yolk Cells. Enclosed in secretions from Mehlis' gland and the vitellaria, tough eggshells are formed, and the complex eggs enter the uterus, gradually moving toward the genital cloaca and out into the environment. Excess spermatozoa and yolk cells are eliminated through Laurer's canal, which opens dorsally.

Reproduction. Fertilization is generally cross-fertilization. All flukes have a complex life cycle characterized by ALTERNATION OF GENERATIONS, parasitic and free-living developmental phases, and a change of hosts (Fig. 124).

Fig. 124. Generalized diagram of the trematode life cycle:

a - parasite in the definitive host; b - in the external environment; c - in the first intermediate host; d - In the second intermediate host

Trematodes are highly prolific, producing a new egg every 20-30 seconds. Typically, the egg must reach Water, where it hatches into a motile larva, the miracidium (Fig. 125). It is covered with cilia and features a proboscis with a penetration organ, eyes, and chemoreceptive sensilla at its anterior end. Large Germ Cells are located in the posterior part of the miracidium's body. For further development, the miracidium must infect an intermediate host, which for trematodes is a specific species of mollusk, predominantly from the Class Gastropoda.

Fig. 125. STRUCTURE OF THE Fasciola hepatica miracidium (after Ginetsinskaya):

1 - retracted proboscis; 2 - apical gland (penetration organ); 3 - cilia;

4 - sensilla; 5 - cerebral ganglion; 6 - cyrtocyte; 7 - protonephridial canal; 8 - bladder; 9 - germ cells; 10 - muscle layer; 11 - developing embryos; 12 - eyespot; 13 - glandular cell

The miracidium searches for an intermediate host and actively penetrates its body through the integument, or the host ingests the egg containing the miracidium. Inside the intermediate host, it typically migrates to the digestive gland, sheds its ciliated epithelium, and transforms into a sac-like, sexually mature mother sporocyst (Fig. 126) of much simpler structure. It loses its eyes, proboscis, and penetration organ. Instead of ciliated epithelium, a tegument forms in the sporocyst. Its body is filled with diploid parthenogenetic eggs that give rise to the second parthenogenetic generation. This generation may be represented by two forms: daughter sporocysts or rediae. The Emergence of the daughter sporocysts and rediae—which are quite motile—from the mother sporocyst occurs through ruptures in its body wall. Daughter sporocysts are sac-like, lack a digestive system, and feed through the tegument. Rediae are elongated, possessing an oral sucker, intestine, and protonephridia; they actively feed on mollusk tissues. Reproducing parthenogenetically, daughter sporocysts and rediae can initiate another generation of daughter sporocysts or progress to the next stage, the cercariae (Fig. 127). In structure, cercariae closely resemble maritae (being the larvae of maritae), but differ in smaller size and the presence of a muscular tail used for swimming. In species with a single intermediate host, cercariae emerge from the mollusk into the water, either settling on a substrate to encyst and transform into adolescariae, or actively penetrating the definitive host's body.

Fig. 126. Sporocysts (after Ginetsinskaya): a - sac-like; b - filiform: 1 - germ balls; 2 - cercariae; 3 - rediae

Fig. 127. Structure of rediae and cercariae (after Ginetsinskaya): a, b - typical rediae; c - sac-like redia; d-f - cercariae:

1 - pharynx; 2 - intestine; 3 - locomotor outgrowths; 4 - rudimentary intestine; 5 - cercarial embryos;

6 - fully formed cercariae; 7 - tail appendage

In many species, further development takes place in a second intermediate host, which may be fish, aquatic insect larvae, other Mollusks, crustaceans, etc. Cercariae are either swallowed by the second intermediate host or actively penetrate its integument using specialized adaptations (stylets, spined proboscides, etc.). Inside the second intermediate host, they encyst in internal organs or muscles, transforming into metacercariae. Subsequently, along with the second intermediate host, they are eaten by the definitive vertebrate host, in whose intestine they develop into maritae. Thus, the trematode life cycle alternates between one hermaphroditic generation (marita) and several parthenogenetic generations (sporocysts, rediae). This type of cycle is known as heterogony.

Life cycles of the best-known trematode parasites affecting humans and domestic animals. The pathogenic impact of trematodes on the host Organism varies. These parasites damage tissue integrity, obstruct the ducts of certain organs, and negatively affect the host by releasing toxic metabolic wastes. Heavy infections (infestations) lead to systemic intoxication of the host with parasite metabolites. Furthermore, some trematodes may promote The Development of various neoplasms, including malignant ones.

The cat liver fluke (Opisthorchis felineus) (Fig. 128), which causes the dangerous disease opisthorchiasis, is distributed worldwide. In the definitive host (humans and a wide range of mammals), the parasite localizes in the intrahepatic Bile ducts and Gallbladder. For further development, the parasite's eggs must reach water, where they are ingested by the intermediate hosts—freshwater gastropods known as Bithynia leachi. The cercariae emerging from the snail are strong swimmers and must penetrate the body of a second intermediate host (cyprinid fish such as bream, roach, and common carp), encysting within their musculature. The definitive host becomes infected by consuming fish containing the parasite's metacercariae. The lifespan of the cat liver fluke spans several years. In Ukraine, opisthorchiasis occurs in the basins of the Dnieper, Seym, Southern Bug, and Danube rivers, among others.

Fig. 128. Life Cycle of Opisthorchis felineus:

a - definitive hosts; b - marita (adult); c - egg containing a miracidium; d - first intermediate host, the snail Bithynia leachi; e - sporocyst; f - redia;

g - cercaria; h - second intermediate host; i — metacercaria: 1 - oral sucker; 2 - pharynx; 3 - intestine;

4 - copulatory organ; 5 - ventral sucker; 6 - vitellaria; 7 - uterus; 8 - ootype;

9 - seminal receptacle; 10 - ovary; 11 - testes; 12 - excretory bladder

Schistosomiasis, affecting hundreds of millions of people worldwide, is one of the most prevalent helminthiases on our planet. The causative agents of these diseases are three species of the genus Schistosoma: Sch. mansoni, Sch. haematobium, and Sch. japonicum. The first two species are human-specific parasites, whereas the third also infects domestic animals (horses, dogs, pigs, etc.). A characteristic feature of schistosomes is dioeciousness accompanied by pronounced Sexual Dimorphism: males have a broader body with lateral margins folded ventrally to form a specialized canal that houses the slender, elongated female (Fig. 129).

Fig. 129. Life cycle of Schistosoma haematobium:

a - adult male and female; b - definitive host; c - miracidium; d - sporocyst; e - redia; f - intermediate host;

g - cercaria; h - cercaria in waiting posture

Adult schistosomes (maritae) parasitize the Veins OF THE intestine or the Urogenital System. Females deposit eggs (up to 160–190 eggs per day) into the lumen of capillaries. Equipped with sharp spines, the eggs are subsequently forced through tissues into the lumen of the intestine or urinary bladder and excreted into the environment. To continue their development, they must reach water. Various species of freshwater snails belonging to the families Planorbidae, Bulinidae, and Hydrobiidae serve as intermediate hosts. Cercariae emerge from the snail into the water. Infection of the definitive host occurs during swimming or by drinking raw water. Upon contacting a host, the cercariae rapidly attach and penetrate the skin or mucous membranes within 2–15 minutes, entering the Blood Vessels. Carried by the bloodstream, they reach the veins of the intestines or urogenital organs, where they mature sexually. Schistosomes can exert a significant pathogenic effect on the host organism. Treatment is challenging, and affected individuals often suffer prolonged or even permanent disability.

Human schistosomiasis is a disease typical of southern and tropical regions. It does not occur in Ukraine; however, individuals bathing in stagnant or sluggishly flowing water may experience schistosome dermatitis, commonly known as swimmer's itch. This condition is caused by schistosome cercariae that parasitize waterfowl. Upon penetrating human skin, the cercariae perish, and the skin itching subsides rapidly.

The liver fluke (Fasciola hepatica), which causes fascioliasis (Fig. 130), is a widespread parasite affecting domestic herbivores (cattle, sheep, goats, pigs, horses, rabbits, etc.). For the liver fluke, humans act as facultative (accidental) definitive hosts, meaning they are not essential for completing The life cycle and lack obligatory ecological ties with the parasite.

The liver fluke possesses a leaf-shaped body up to 30 mm in length. Adult specimens localize primarily in the bile ducts of The Liver and the gallbladder, and less frequently in the lungs. Fluke eggs pass with bile into the intestine and are subsequently excreted outwards. In water, a miracidium hatches from the egg and must penetrate the intermediate host, the dwarf pond snail (Lymnaea truncatula). Within the snail, the parasite undergoes the stages of a mother sporocyst, several generations of rediae, and cercariae. Upon emerging from the snail into the water, the cercariae attach to aquatic plants or the water surface film, shed their tails, and encyst. The encysted adolescent stage, or metacercaria (adolescaria), can withstand desiccation for several months. The parasite enters

the definitive host via raw water or ingestion of plants bearing encysted metacercariae.

Fig. 130. Life cycle of Fasciola hepatica:

a - marita; b - fertilized egg; c - hatching of miracidium from the egg; d - miracidium; e - young mother sporocyst; f - mature sporocyst; g - intermediate host, the snail Limnaea truncatula; h - redia; i - cercaria; j - adolescent metacercaria (adolescaria) on grass; k - definitive hosts:

1 - oral sucker; 2 - genital atrium; 3 - copulatory organ; 4 - ejaculatory duct; 5 - uterus; 6 - Mehlis' gland;

7 - vitelline ducts; 8 - vitellaria; 9 - sperm duct (vas deferens); 10 - Testis; 11 - oviduct; 12 - ovary;

13 — ventral sucker; 14 - intestine; 15 - pharynx; 16 - penetration gland; 17 - eyespot (oculus); 18 - cilia; 19 - germ cells;

20 - embryonic balls; 21 - mother redia; 22 - daughter redia; 23 — tail

Dicroceliosis is diagnosed in humans less frequently than fascioliasis. The causative agent is the lancet liver fluke (Dicrocoelium dendriticum), which parasitizes primarily the bile ducts of cattle, small ruminants, horses, hares, rabbits, bears, and other mammals. The parasite's body is lanceolate and up to 15 mm in length (Fig. 131). Its life cycle is not linked to an aquatic environment: the eggs are ingested by terrestrial gastropods (species of the genera Helicella, Zebrina, Torquilla, Theba, etc.), which serve as the first intermediate host. Inside their intestine, a miracidium hatches from the egg and penetrates the mollusk's digestive gland, transforming into a mother sporocyst that gives rise to numerous daughter sporocysts. Cercariae emerging from the daughter sporocysts actively penetrate the pulmonary cavity of the mollusk, where they encyst. The encysted cercariae are glued together by mucus into specific "slime balls" (collective cysts), which are expelled outward during the mollusk's respiratory movements and end up On the surface of plants. The outer layer of mucus dries and becomes dense, protecting the cercariae inside the cyst so they remain viable for a long time. The second intermediate host is ants of the genus Formica, which ingest the collective cysts. In the ant's intestine, the cercariae shed the mucus, migrate to the musculature, and encyst to become metacercariae. However, one of the cercariae penetrates the ant's Brain and, remaining unencysted, induces profound behavioral Changes in the insect. An infected worker ant performs its usual functions throughout the day, but toward evening, when the air Temperature drops, it clamps its mandibles onto a blade of grass and hangs suspended, losing The ability to move until morning. Near heavily infected anthills at this time of day, many ants can be seen hanging motionless on the grass. Sheep or other herbivores swallow the ants along with the vegetation, thereby becoming infected. Humans can become infected by accidentally swallowing an infected ant; like F. hepatica, humans serve only as an accidental (facultative) definitive host for D. dendriticum.

Fig. 131. Life cycle of Dicrocoelium dendriticum:

a – marita (adult fluke); b – egg with miracidium; c – sporocyst; d – first intermediate host – terrestrial mollusk; e – mucous collective cyst with cercariae; f – cercaria; g – metacercariae in the brain and body of the second intermediate host – ant; h – invaded ant on grass;

i – definitive hosts: 1 – oral sucker; 2 – pharynx; 3 – intestine; 4 – ventral sucker; 5 – testes; 6 – vitellaria (yolk glands); 7 – uterus; 8 – ovary; 9 – ejaculatory duct; 10 – copulatory organ (cirrus); 11 – genital pore