INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM MULTICELLULAR ANIMALS (METAZOA)

SUBKINGDOM TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

DIVISION TRIPLOBLASTIC (TRIPLOBLASTICA) OR BILATERAL (BILATERIA) ANIMALS

SUBDIVISION SPIRALIA (SPIRALIA)

PHYLUM THORNY-HEADED WORMS (ACANTHOCEPHALA)

Specialized parasites of the intestine in various vertebrates. More than 1,500 species are known, with about 60 found in Ukraine.

The anterior end of the body features a retractable proboscis armed with hooks. The body wall is formed by a musculocutaneous sac; its superficial layer, the cuticle, is penetrated by pores, and the hypodermis is complex in Structure and contains lacunae. The schizocoel is well developed. The digestive tract is absent at all Selection/3.html">Stages of development. The excretory system is protonephridial. The Nervous System is an orthogon; Sensory Organs are absent. Thorny-headed worms are dioecious. Development occurs with metamorphosis. Specific organs of Acanthocephala include a fibro-muscular sac, the ligament, and hypodermal derivatives known as lemnisci.

The size of thorny-headed worms ranges from 1.5 mm (Pandasentis iracundus) to 68 cm (the giant thorny-headed worm Macracanthorhynchus hirudinaceus). These animals are typically whitish in color, occasionally displaying bright orange, yellow, coral, or brown pigmentation.

Structure. The body of thorny-headed worms is generally elongated and cylindrical, consisting of a proboscis, neck, and trunk. The proboscis serves as the primary attachment organ. It bears cuticular hooks arranged in regular rows, which the worms use to anchor themselves in the host's intestinal wall. The proboscis can be retracted into a sheath (Fig. 154).

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Fig. 154. Diagram of The structure of Acanthocephalus lucii (from Strelkov):

a - female; b - male; c - uterine bell, enlarged; d - proboscis:

1 - proboscis; 2 - proboscis sheath; 3 - lemnisci; 4 - Muscles; 5 - ligament;

6 - eggs and egg balls; 7 - uterine bell; 8 - Uterus; 9 - Vagina; 10 - genital opening; 11 - cement glands;

12 - copulatory bursa; 13 - copulatory organ; 14 - Testis; 15 - anterior opening of the bell; 16 - wall of the bell;

17 - slit-like opening of the bell; 18 - mature egg; 19 - oviduct

The musculocutaneous sac consists of an epicuticle, cuticle, hypodermis, and two layers of smooth muscles. The epicuticle is a thin film that protects the parasites from the host's Enzymes and immune responses. The cuticle is a thin layer covered by a Cell/33.html">Plasma Membrane and pierced by numerous pores, from which tiny canaliculi extend into the hypodermis. It performs protective and nutrient-absorption Functions. The hypodermis, the thickest layer of the body wall, has a syncytial structure and contains numerous supporting fibrils as well as a complex system of lacunae through which nutrients absorbed across the entire body surface are processed and transported. The reserve substance, Glycogen, is stored in the hypodermis.

Except for the proboscis retractors, the musculature is poorly developed: thorny-headed worms are sluggish animals.

The body cavity is a primary body cavity (schizocoel). It is filled with fluid that performs a transport and Distribution Function, carrying nutrients from the hypodermis to various PARTS OF THE body, and metabolic end products in the opposite direction. At the anterior end of the body, the proboscis sheath—a muscular sac—projects into the schizocoel; inside this sac, special retractor muscles pull the proboscis inward, much like turning a glove finger inside out. On either side of the proboscis sheath lie specific organs called lemnisci, which are invaginated hypodermal outgrowths projecting into the body cavity and containing large lacunae. Their function is to regulate pressure within the proboscis sheath during the eversion of the proboscis. It is also believed that the lemnisci, like the hypodermis, form part of the transport system.

The Digestive System is absent. As already mentioned, the function of nutrient absorption and Processing is carried out by the body wall.

Excretory organs, or protonephridia, are found only in certain Representatives of the phylum. They appear as capsules containing numerous flask-shaped cyrtocytes or as a branching, tree-like organ (Fig. 155). The main excretory duct of the protonephridia merges with the ducts of the Reproductive System to form a common urogenital cloaca.

Fig. 155. Excretory system of thorny-headed worms (from Dobrovolsky): a - capsule-like organ; b - dendritic protonephridium:

1 - flask-shaped cyrtocytes; 2 - nephridial capsule; 3 - excretory duct

The nervous system is orthogono-type. It is represented by an unpaired cerebral ganglion located inside the proboscis sheath, from which two lateral nerve trunks extend, giving off branches to the body wall and various organs (Fig. 156). Nerves to the proboscis retractor muscles and its receptors (sensory papillae) also depart from the cerebral ganglion. In males, there is additionally a genital ganglion that innervates the copulatory apparatus and genital papillae, whereas females possess a cluster of Nerve Cells near the genital opening.

Fig. 156. Diagram of the nervous system structure in the anterior end of the spiny-headed worm Polymorphus phippsi (from Dobrovolsky):

1 - apical sensory organ; 2 - proboscis nerves; 3 - proboscis sheath nerves; 4 - anterior median nerve trunk;

5 - wall of the proboscis sheath; 6 - central (main) ganglion; 7 - lateral body wall nerves

Sense Organs in acanthocephalans, as in many endoparasites, are poorly developed. They are mostly sensory papillae located at the tip of the proboscis, near its base, and also close to the genital opening in the posterior part of the body. Only the proboscis responds to various stimuli by retracting into its sheath.

Reproductive system. A distinctive feature of the acanthocephalan reproductive system is its connection to the ligament, which attaches to the bottom of the proboscis sheath and extends through the body cavity to the posterior end.

The FEMALE REPRODUCTIVE SYSTEM has a rather peculiar structure. The rudiments of one or two Ovaries are located within the ligament in young females; over time, they break down into separate egg clusters—groups of oocytes that float freely in the body cavity once the ligament walls rupture. Spermatozoa also enter here after mating. Fertilized eggs develop within the body cavity, while mature ones become narrow and spindle-shaped. They are expelled through an organ characteristic exclusively of acanthocephalan worms—the uterine bell (Fig. 154, c), which sorts mature and immature eggs in a unique way. It opens broadly into the body cavity, while its opposite end features two openings: one, larger, opens into the body cavity, and another, narrower one, leads to the efferent genital ducts. Rounded immature eggs and egg clusters return to the body cavity through the wide opening, whereas spindle-shaped mature eggs containing larvae enter the genital ducts and are discharged outside through the narrower one.

The Male Reproductive System consists of two Testes, vas deferens, and a vas deferens duct whose terminal section—the ejaculatory duct—penetrates the copulatory organ, which opens into the bursa copulatrix. Cement glands also open into the ejaculatory duct, secreting a substance to seal the female's genital opening after mating.

Reproduction. Fertilization is internal. The laid eggs contain a larva, the acanthor, which is passed out with the host's excrement. The acanthor hatches from the egg only inside the body of an intermediate host—crustaceans or insect larvae—where it develops into the next stage, the acanthella (Fig. 157). It differs from adult worms only in size and underdeveloped reproductive organs. A fully formed acanthella retracts its proboscis and forms a cyst; this stage is known as the cystacanth. The definitive host is a vertebrate animal.

Fig. 157. Larval stages of acanthocephalans (from Petrochenko):

a - acanthor; b - acanthella; c - cystacanth

The life cycle involves two obligate hosts—an intermediate and a definitive one—but can be complicated by the participation of paratenic (reservoir) hosts. For instance, in species of the genus *Corynosoma*, the definitive hosts are seals, and the intermediate hosts are amphipods. Fish that feed on crustaceans can serve as reservoir intermediate hosts. In these fish, the acanthocephalan larvae reside in the body cavity or muscles in an inactive state, developing further only after being ingested along with the fish into the seal's intestine.

Acanthocephalans are capable of causing tangible harm to the host Organism: with their proboscis, they can penetrate into the muscular and even serous layer of the intestinal wall, causing inflammation, ulceration, and tissue necrosis around the site of proboscis penetration. Heavy infections may lead to rupture of the intestinal wall and, consequently, Peritonitis. In addition, the parasites exert a toxic effect on the host organism.

Severe damage to pig farming (including in Ukraine) is caused by the giant thorn-headed worm (*Macracanthorhynchus hirudinaceus*), which parasitizes the intestines of domestic and wild pigs, and occasionally humans. Females reach up to 80 cm in length, laying between 82,000 and 580,000 eggs daily.

Lifespan in the definitive host ranges from 15 to 23 months. Adult pigs are most commonly affected, becoming infected by feeding on intermediate hosts—larvae, pupae, and adult beetles (chafers, scarabs, and more rarely dung beetles and ground beetles).

The acanthocephalan *Echinorhynchus polymorphus* frequently parasitizes the intestines of domestic waterfowl. Their intermediate hosts are crustaceans.

Isolated cases of acanthocephalan parasitism in humans have been documented. Infection occurs only when a person swallows an insect or eats raw fish containing acanthocephalan larvae that normally parasitize wild mammals.