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 TAPEWORMS (CESTODA)

This Class comprises about 3,500 species of endoparasites (over 500 species are known in Ukraine) that parasitize vertebrates during their adult stage (with the exception of Representatives of the genus Archigetes, which live in the body cavity of oligochaete worms), while their larval stage typically inhabits invertebrates, particularly Arthropods. In some species, larvae also parasitize vertebrates, including humans.

While the external Morphology of adult tapeworms varies, their general body plan remains consistent across all representatives of the class. The body is divided into a HEAD, or scolex, followed by an unsegmented neck that serves as a "growth zone," behind which segments, or proglottids, bud off to form the strobila. Some tapeworms are single-segmented, whereas others consist of two to four, or even thousands of proglottids.

The size of tapeworms ranges from a few millimeters to 10 meters. Representatives of the family Diphyllobothriidae include giants exceeding 20 meters in length. Because adult worms primarily inhabit the intestine and must resist its peristalsis, the scolex bears specialized attachment Organs that secure the parasite firmly in place. The shape of the head can be rounded, elongated, flattened, etc. Their attachment organs are diverse, taking the form of sucker-like grooves (bothria) or complex bothridia formed by elongated suckers divided by numerous septa (Fig. 161). Most commonly, the scolex bears four, and occasionally one or five, powerful cup-shaped suckers, while some species additionally possess hooks of various structures located on the head or, as in representatives of the genus Tetrarhynchus, on four long proboscides. In certain tapeworms, the anterior part of the body expands to form a folded rosette, which the parasite uses to anchor itself to the host's intestinal wall.

Fig. 161. Attachment organs of tapeworms — Diphyllobothrium latum (a), Taeniarhynchus saginatus (b), Caryophyllaeus laticeps (c), Taenia solium (d), Echinobothrium sp. (e), Tetrarhynchus sp. (f): 1 — bothria; 2 — suckers; 3 — festoons; 4 — bothridia; 5 — proboscides with hooks; 6 — proboscis sheaths

The next body region is the neck. It is short and relatively narrow, with the proglottid budding zone located at its posterior end. The number of segments in polyzoic species varies and depends on the physiological state of the parasite. New proglottids are formed either for a relatively short period or almost throughout the entire life of the parasite. The oldest proglottids, situated at the posterior part of the strobila, may detach singly or in groups of five to six. They can actively move and remain in the intestine for some time before being expelled into the external environment.

The integument of tapeworms differs somewhat from that of other Flatworms (Fig. 162). Since tapeworms lack a Digestive System, their integument performs a nutritional function. The outer surface of the tegument is densely covered with microvilli—microtriches of two types: cone-shaped and tubular. The cone-shaped microtriches are located at the sites of contact between the parasite and the host's intestinal walls. Based on their structural features, these microvilli are believed to be involved in the attachment and locomotion of the parasite.

Tubular microtriches perform a trophic function; their Structure and arrangement On the surface of the strobila resemble the brush border of the vertebrate intestinal epithelium.

Fig. 162. Diagram of the tapeworm integument structure (based on Electron Cell/15.html">Microscopy data):

1 — microtriches; 2 — pore canal; 3 — tegument; 4 — Mitochondria; 5 — basal membrane; 6 — circular Muscles; 7 — longitudinal muscles; 8 — cytoplasmic strands; 9 — Endoplasmic reticulum; 10 — submerged region of Cytoplasm; 11 — Nucleus; 12 — protein and lipid inclusions; 13 — Glycogen inclusions

The muscular component of the Skin-muscular sac is represented by circular and strongly developed longitudinal muscles. Additionally, the parenchyma of tapeworms contains an internal circular layer, and many species also possess bundles of dorsoventral muscles.

In the peripheral layers of the parenchyma, alongside the submerged PARTS OF THE tegument Cells, there are calcareous cells in which " calcareous corpuscles" are formed. Their composition includes Proteins, CARBOHYDRATES, Lipids, and mineral components (calcium carbonate, magnesium carbonate, and a small fraction of phosphorus). In many tapeworms, these calcareous corpuscles are already formed during larval stages (procercoid, plerocercoid). It is believed that they are utilized by the parasite to neutralize the acidic environment, which is especially important when the larvae enter The Stomach.

The excretory, nervous, and reproductive systems are located within the parenchyma. The excretory system of tapeworms is of the protonephridial type, with numerous cyrtocytes connected via fine channels to four main longitudinal canals. Two channels originate at the posterior end of the strobila, extend along the dorsal side to the scolex, form a rather complex loop there, and return along the ventral side toward the posterior end, where they may merge into a Urinary Bladder. In species whose proglottids do not detach from the strobila, the bladder persists throughout life. Upon the shedding of segments, each of the four canals opens externally via its own pore. In many species, the longitudinal trunks are connected by transverse anastomoses.

The Nervous system of tapeworms belongs to the orthogon type; specialized Sense Organs of complex structure are absent.

Sensilla are sparsely scattered across The surface of the strobila, forming significant aggregations only on the scolex.

The Reproductive System of tapeworms, like that of most flatworms, is hermaphroditic and complex in structure (Fig. 163). Only representatives of the order Caryophyllidea possess a single set of reproductive organs. All other tapeworms have one, or more rarely two, sets in each segment. The degree of Development of the reproductive organs depends on THE POSITION OF the proglottid relative to the neck. In young segments, the reproductive system differentiates and develops; the middle region of the strobila consists of segments with a fully developed reproductive system. At the posterior end of the strobila are gravid (overripe) segments containing a heavily branched Uterus packed with eggs, alongside remnants of the reduced reproductive apparatus.

Fig. 163. Diagram of the tapeworm reproductive system — Diphyllobothrium latum (a), Taeniarhynchus saginatus (b): 1 — genital atrium; 2 — copulatory organ (cirrus); 3 — uterine pore; 4 — vas deferens; 5 — Testes; 6 — uterus; 7 — vitelline glands (vitellaria); 8 — ootype surrounded by Mehlis' gland; 9 — vitelline ducts; 10 — oviduct; 11 — Ovary; 12 — seminal receptacle (enlargement of Vagina); 13 — vagina; 14 — sperm duct

The Male Reproductive System consists of testes (ranging from one to several hundred) from which vasa efferentia depart. Joining together, they form the vas deferens, the terminal part of which Functions as a copulatory organ, the cirrus. It is enclosed within a muscular pouch that opens into the genital atrium. The FEMALE REPRODUCTIVE SYSTEM is even more complex. Its main component is the ovary, from which an oviduct leads into the ootype. The ducts of the vitelline glands and the shell gland (Mehlis' gland) also empty into the ootype. Fertilization takes place within the ootype, completing The formation of eggs supplied by the ovary.

The ootype gives rise to the uterus and vagina, the latter opening at its other end into the genital cloaca. In some cestodes (order Pseudophyllidea), the uterus has an external opening (an open uterus), whereas in others (order Cyclophyllidea), the uterus lacks such an opening. In the former case, maturing eggs are released into the external environment; in the latter, they are shed through the rupture of the proglottid and uterine walls. This process typically occurs in the external environment, where the mature proglottids end up.

Cestodes develop through a change of hosts: some species have two hosts (a definitive and an intermediate one), while others have three (the definitive host plus two intermediates). Cestodes undergo cross-fertilization or self-fertilization between different proglottids or within the same proglottid when the cirrus is inserted into the vagina of the same segment.

The larva developing within the fertilized egg is uniform across all tapeworms and is referred to as a hexacanth embryo, or oncosphere. The larva is spherical or oval in shape, with six chitinoid hooks at its posterior end that are moved by means of Muscle cells (Fig. 164).

Fig. 164. Types of cestode larvae:

a — oncosphere; b — cysticercus; c — coenurus; d — echinococcus; e — plerocercoid; f — procercoid; g — coracidium; h — cysticercoid

Large glandular cells are situated at the anterior end of the oncosphere, the secretion of which facilitates the movement of the oncosphere through the intermediate host's body. The anterior hemisphere contains 'embryonic' cells, which give rise to the body of subsequent larval stages. For further development, the oncosphere must enter the body of an intermediate host, where larvae of various types develop; their structure will be examined below using specific Examples.

Authors distinguish anywhere from four to eleven orders within the class of tapeworms; we shall consider the most typical representatives, specifically those species that cause severe and sometimes life-threatening diseases in their hosts known as cestodiases.

Order Pseudophyllidea

This order includes both unsegmented forms (possessing multiple sets of reproductive organs) and segmented forms. Among the segmented forms are the longest helminths. For instance, Polygonoporus giganticus, which parasitizes the intestine of the sperm whale, reaches nearly 30 m in length. The scolex in pseudophyllideans typically bears two bothria, occasionally supplemented with shallow sucking pits or hooks. The Development of pseudophyllideans involves two intermediate hosts: the first being various species of crustaceans, and the second, fish. Definitive hosts can be representatives of various vertebrate classes.

Belonging to the family of broad tapeworms is Diphyllobothrium latum, widely distributed in northern Eurasia and North America, using which we will examine The life cycle of tapeworms in greater detail (Fig. 165). Its adult specimens parasitize many carnivorous mammals and humans. Usually, a single specimen, or more rarely two, parasitizes the human intestine. The tapeworm's body consists of several thousand proglottids and can reach a length of 10–15 m (with a width of 15 mm). D. latum is easily distinguished from other species by its large, dark, rosette-shaped uterus present in every mature proglottid. The uterus is open, and for development to proceed, the eggs must reach Water. A free-swimming larva—the coracidium, an oncosphere covered with ciliated epithelium—hatches from the egg (see Fig. 164). If swallowed by a copepod crustacea of the order Copepoda, the oncosphere transforms into a procercoid in its body cavity (see Fig. 164). In the process, the larva loses its regular spherical shape and elongates; its posterior end, bearing the hooks, becomes demarcated from the body by a narrow constriction. This hook-bearing region is called the cercomer. Simultaneously, a complex reorganization of the tegument begins: the ciliated epithelium is shed and replaced by a tegument that persists throughout the worm's life. The number of protonephridia increases, and unicellular glands develop at the anterior end of the procercoid. Numerous ' calcareous' corpuscles appear in the surface layers of the parenchyma.

Fig. 165. Life Cycle of Diphyllobothrium latum — scolex, cross-section of the scolex (b), mature proglottid (c), egg (d), egg with a formed coracidium (e), coracidium (f, g), plerocercoid (h), definitive hosts (i), first intermediate host — cyclops (j), second intermediate host — fish (k): 1 — bothrium; 2 — uterus with eggs; 3 — cercomer

Further development of the parasite is possible only within a second intermediate host, which can be various species of river fish that feed on crustaceans. The crustaceans are digested, while the procercoid migrates to various Internal Organs and Muscles of the host, where it transforms into a plerocercoid (see Fig. 164). This transformation is accompanied by the loss of the cercomer and rapid growth of the parasite, which sometimes reaches 1–5 cm in length. Rudiments of bothria appear at the anterior end of the plerocercoid. The life cycle can be complicated by the involvement of reservoir hosts—larger predatory fish that feed on smaller ones—in whose bodies the plerocercoid remains viable and likewise ends up in various organs. Plerocercoids retain viability for a long time In the second intermediate and reservoir hosts. They enter the definitive host, which, alongside humans, may include a range of domestic and wild animals, when the host feeds on infected fish. The most persistent foci of human diphyllobothriasis occur in areas where people consume semi-raw or sometimes raw fish, lightly salted pike caviar, etc. It is known that the broad tapeworm can live in a host's body for up to 20 years.

The order Pseudophyllidea also includes the family Ligulidae, among which Ligula intestinalis (the common ligula or strap tapeworm) is a very widespread species, particularly in Ukraine (Fig. 166). Sexually mature unsegmented specimens inhabit the intestines of various fish-eating birds, which disperse vast quantities of eggs over water bodies. Coracidia emerge from the eggs in water and are eaten by copepods, inside which they develop into procercoids. Along with the crustaceans, the procercoids enter the second intermediate host—various species of cyprinid fish. From the fish intestine, the procercoids migrate into the body cavity, where they grow intensively and transform into plerocercoids that are nearly as large as adult ligulas, developing almost all organs, including the reproductive system. As the plerocercoids develop, the fish's abdomen swells, causing it to lose body control; it lingers near the water surface and becomes an easy prey for piscivorous birds.

Fig. 166. Life cycle of Ligula intestinalis: a — sexually mature specimen; b — definitive host; c — egg with embryo; d — coracidia; e — procercoid; f — first intermediate host; g — plerocercoid; h — second intermediate host

Sometimes the fish's abdomen ruptures under the pressure of the growing plerocercoids, resulting in the fish's death. In the bird's intestine, the worms rapidly reach sexual maturity. Ligulas begin intensive egg production and die within two to four days. Ligulosis causes severe damage to fisheries.

Order Cyclophyllidea

This order comprises the majority of cestodes, which are predominantly parasites of birds and mammals. Several species parasitize humans. Development mostly proceeds with a single intermediate host. In some tapeworms, asexual reproduction occurs during larval stages, i.e., metagenesis—the alternation of two generations: sexual and asexual. Unlike pseudophyllideans, cyclophyllideans have lost the free-swimming larval stage; their egg contains an oncosphere—already formed within the uterus—enveloped in embryonic membranes. This oncosphere retains viability for a prolonged period while eggs reside in damp soil, litter, or animal feces.

Intermediate hosts of tapeworms can include Mollusks, Annelids, crustaceans, insects, and representatives of all vertebrate classes. Within the intermediate host, the oncosphere sheds its membranes, penetrates the intestinal wall into the host's bloodstream, and subsequently settles in its organs. Here, the transformation of the oncosphere into a larvocyst (the next stage) begins. The structure of these larvae within the order is quite diverse. The most primitive larvocysts are cysticercoids, which feature a bladder-like body with a long tail appendage (cercomer) bearing three pairs of embryonic hooks (see Fig. 164). A scolex, inverted inward, forms at the anterior end of the bladder.

Significantly more common in tapeworms are larvocysts known as bladders or cysticerci. They possess varying structures and differ in the degree of bladder development and the number of scoleces. They lack a distinct separate cercomer. The simplest in structure is the cysticercus-type larva. The mature larva acquires a spherical or oval shape with a large internal cavity. Three pairs of larval hooks are retained at one pole, while a deep invagination appears at the opposite pole, at the bottom of which a single scolex forms (see Fig. 164). Coenurus and echinococcus types of bladders possess a more complex structure. In the former case, multiple rather than single scoleces develop on the inner surface of the bladder, meaning more parasites develop from a single oncosphere (see Fig. 164). Echinococci are larvocysts that sometimes attain considerable size. Externally, they are covered by a capsule formed by the host's Tissues, while the bladder walls proper consist of several layers. The inner layer proliferates to form brood capsules where scoleces arise; daughter bladders, upon which scoleces also form, bud off from this layer. Thus, a vast number of scoleces develop inside the echinococcus (see Fig. 164). This process is viewed as asexual reproduction at the larval stage.

In the definitive host's intestine, under the action of digestive Enzymes, the scolex evaginates, and the cercomer or the portion of the bladder bearing the larval hooks is discarded.

Of greatest practical importance are tapeworms from the family Taeniidae. In humans, taeniids cause dangerous helminthiases that can prove fatal to the patient. The disease is caused by both adult forms and larvae.

The beef tapeworm, or unarmed tapeworm, Taeniarhynchus saginatus (see Figs. 161, 163) parasitizes humans, and in its larval stage, cattle. Adult cestodes reach a length of 4–10 m. Their scolex, as in all tapeworms, bears four large suckers. Characteristically, the blind-ending uterus in mature proglottids has the appearance of a longitudinal stem from which up to three dozen poorly branched lateral branches depart. Overripe proglottids of T. saginatus detach one by one from the strobila and are passed out with feces. For some time, they move across the substrate and scatter eggs. The parasite's eggs reach intermediate hosts via contaminated grass. In the intestine, an oncosphere emerges from the egg, penetrates Blood Vessels, and is carried via blood to various organs. Within their tissues, the oncosphere transforms into a cysticercus-type bladder (the next larval stage). These bladders localize most frequently in muscles, although they have also been found subcutaneously, in the eyes, Brain, and other organs. Larval parasitism in cattle usually proceeds asymptomatically, but if the parasite localizes in the eyes, brain, or other vital organs, cattle mortality is possible, especially among young stock. Humans become infected with the tapeworm by consuming undercooked or rare beef. Patients suffer from insomnia, irritability, and occasionally experience epileptic-like seizures. Children suffer particularly severely.

The pork tapeworm, or armed tapeworm (Taenia solium), is smaller in size (2–3 m, rarely up to 8 m) than the beef tapeworm (Fig. 167). In addition to suckers, its scolex bears a short rostellum with two rows of hooks (hence the name "armed"). The uterus in a mature proglottid resembles that of T. saginatus, although it has fewer lateral branches—7 to 12. The intermediate hosts of this tapeworm are domestic and wild pigs, while the definitive host is humans. The pork tapeworm is characterized by the same developmental cycle as the beef tapeworm, and the disease it causes presents with similar symptoms. However, tapeworms of this species are more dangerous to humans because humans can also serve as their intermediate host, in which case cysticercosis develops. Infection can occur when a person swallows eggs dispersed in the environment, but autoinfection is also common. Typically, proglottids or chains of proglottids are passed with feces, but they may also enter the stomach during vomiting.

Fig. 167. Life cycle of Taenia solium: anterior end of an adult specimen (a), definitive host (b), egg with oncosphere (c), meat products containing cysticerci (d), cysticercus with everted scolex (e), cysticercus (f), intermediate host (g), oncosphere (h), mature proglottid (i): 1 — oncosphere in the intermediate host; 2 — cysticercus in the Organs of the intermediate host; 3 — uterus with eggs

In the stomach, under the action of gastric juice, oncospheres massively hatch from the eggs and are subsequently carried throughout the body. The disease is most severe when cysticerci localize in the BRAIN AND SPINAL cord, resulting in headaches, visual disturbances, hallucinations, memory loss, and progressive deterioration of health. The disease frequently leads to death.

Teniasis and cysticercosis are widespread globally, particularly in tropical regions.

The hydatid tapeworm (Echinococcus granulosus) is one of the most dangerous human parasites. Adult worms parasitize the intestines of dogs, wolves, and other carnivores. The strobila consists of three to four proglottids (Fig. 168). The scolex possesses four suckers and a rostellum armed with two rows of hooks. The mature terminal proglottid detaches from the strobila and is expelled with feces. It retains The ability to move and can crawl a distance of up to 25 cm. The range of intermediate hosts for Echinococcus is exceptionally broad, encompassing all domestic and wild ungulates, marsupials, and certain rodents, which become infected by ingesting eggs or proglottids along with grass, water, etc. In their intestines, oncospheres hatch from the eggs, penetrate the bloodstream, and are carried to various organs, most frequently The Liver and Lungs. Here, the oncosphere develops into the next larval stage—a cysticercus, which forms a hydatid cyst. These cysts can grow from the size of a pea to that of a watermelon. This process takes years; hydatid cysts from cattle livers weighing over 60 kg have been documented. Definitive hosts become infected by consuming the internal organs of infected intermediate hosts.

Fig. 168. Life cycle of Echinococcus granulosus: adult specimen (a), egg with oncosphere (b), intermediate hosts (c), Echinococcus-type cyst (d), definitive host (e): 1 — scolex; 2 — immature proglottid; 3 — hermaphroditic proglottid; 4 — mature proglottid

Humans, who can act as an intermediate host for Echinococcus, most commonly become infected through contact with dogs sick with echinococcosis, whose fur is contaminated with A large number of parasite eggs.

Echinococcosis is a dangerous human disease. When the parasite localizes in the liver, patients experience pain, hepatomegaly, and signs of anemia; when it parasitizes the lungs, symptoms include coughing, shortness of breath, fever, and all the clinical signs associated with tuberculosis. Rupture of the cyst can induce anaphylactic Shock and frequently results in death.

Echinococcosis causes severe economic losses in animal husbandry. The primary factor in its spread is the practice of feeding dogs the internal organs of slaughtered infected animals.

Another notable parasite is the gid parasite (Multiceps multiceps), which causes sturdy-gid or “trotting” disease in sheep. Adult gid worms inhabit the intestines of dogs and other canids, just like Echinococcus. The larval stage, known as a coenurus, localizes in the brain and occasionally the Spinal Cord of sheep. The larval stage of this parasite can also infect humans.

Order Caryophyllidea

This is a small group of tapeworms with an undivided body, a single set of reproductive organs, and a rosette-shaped scolex (Fig. 169). Their development involves a single intermediate host—an oligochaete worm—in the body cavity of which a procercoid develops with a long caudal appendage. The adult parasite inhabits the intestine of cyprinid fish. Consequently, the developmental cycle of caryophyllideans lacks plerocercoid stages, which some scientists view as an example of a primarily simplified life cycle characteristic of ancestral tapeworm groups.

Fig. 169. Caryophyllaeus laticeps: general view (a), structural details of the posterior body end (b): 1 — scolex; 2 — testes; 3 — vitellaria; 4 — vas deferens; 5 — cirrus pouch; 6 — ovary; 7 — cirrus (copulatory organ);

8 — female genital pore; 9 — vagina; 10 — seminal receptacle; 11 — Mehlis' gland; 12 — uterus; 13 — oviduct; 14 — male genital pore

Other researchers consider this phenomenon to be neoteny—larval reproduction occurring at the plerocercoid stage.

This order also includes Archigetes, a parasite of oligochaete worms that develops without an intermediate host (Fig. 170). It retains its cercomer throughout its entire life. On this basis, it is regarded as a neotenic larva, specifically a procercoid, meaning that the plerocercoid and adult stages have been omitted from its life cycle.

Fig. 170. Life cycle of Archigetes sieboldi: specimen with mature eggs emerged from the host (earthworm) (a), dead specimen shedding eggs (b), egg (c), egg with oncosphere (d), sexually mature specimen (e), earthworm with parasites (f): 1 — bothrium; 2 — uterus with eggs; 3 — cercomer with hooks

There are two hypotheses regarding The Nature of the tapeworm strobila. According to the first, the tapeworm body arose As a result of incomplete asexual reproduction; that is, it represents a colony of polymorphic individuals formed via transverse division, or paratomy. This is known as the polyzoic hypothesis of tapeworms. Among tapeworms, there are fish parasites in which proglottids form via paratomy. Each proglottid can form secondary proglottids, after which the strobila breaks apart into individual daughter strobilae, and a new scolex develops at their anterior end.

The second hypothesis asserts that the segmented body of tapeworms is not a colony, but a unified Organism (the monozoic hypothesis). Its proponents emphasize the Organization OF THE nervous and excretory systems, which integrate all body regions into a single whole.