INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION TRUE MULTICELLULAR ANIMALS (EUMETAZOA)

PHYLUM THORNY-HEADED WORMS, OR ACANTHOCEPHALANS (ACANTHOCEPHALA)

CLASS ACANTHOCEPHALA

The body length of acanthocephalans ranges from 1.5 to 650 mm. These animals are whitish, sometimes bright orange or brown in color. Their body has an elongated, cylindrical shape and consists of a proboscis, a neck, and a trunk (Fig. 179). The proboscis is the primary attachment organ in acanthocephalans. In various species, it may be cylindrical, oval, or spherical. Throughout its length, cuticular hooks are arranged in longitudinal or spiral rows. These hooks have a characteristic shape consisting of a ROOT embedded in the hypodermis and a free tip curved backward. When the parasite attaches to the host's intestinal wall, the hooks act on THE PRINCIPLE OF an anchor. The lower part of the proboscis and the neck are devoid of hooks (Fig. 180).

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Fig. 180. Structure of proboscis hooks in acanthocephalans — arrangement of hooks on the proboscis (a), structure of an individual hook (b): 1 — cuticle; 2 — hook tip; 3 — root embedded in Tissues

The Skin-muscular sac of acanthocephalans consists of an epicuticle, a cuticle, and a hypodermis, under which lies a basal membrane and two Muscle layers — circular and longitudinal (Fig. 181).

The epicuticle is a thin film containing mucopolysaccharides. It protects the parasite from the action of the host's digestive Enzymes. The cuticle is a thin layer covered externally by a Cell/33.html">Plasma Membrane and penetrated by numerous pores, from which canaliculi extend into the hypodermis. The cuticle performs protective and absorptive Functions. Through pore canaliculi, nutrient molecules enter the hypodermis, where they are digested. The hypodermis has a syncytial structure. It is the thickest layer of the integument, in which three distinct layers differing in structure and arrangement of supporting fibers can be identified. The fibers are most abundant in the middle layer, where they interlace in various directions. This layer performs the primary structural support function within the integument. The bottom layer contains a complex system of lacunae, appearing as fluid-filled slits between bundles of radial fibers, which house the nuclei of the hypodermal syncytium. This layer concentrates the highest number of Mitochondria and is the site of active metabolic processes. The lacunar System of the hypodermis is connected to tubular Muscles, and its primary function is transport-metabolic. A large amount of the reserve substance Glycogen has been found in the lower layer of the hypodermis.

Fig. 181. Diagram of the integument structure in acanthocephalans (based on Electron Microscopy data):

1 — epicuticle; 2 — cuticle; 3—5 — hypodermal layers; 5 — basal membrane; 6 — circular muscles; 8 — Connective Tissue; 9 — longitudinal muscles; 10 — pores; 11 — plasma membrane; 12 — canaliculi; 13 — hypodermal fibers; 14 — lacunar canals; 15 — lacuna; 16 — hypodermal Nucleus

Thus, acanthocephalans possess a highly peculiar integumentary structure, which is associated with their parasitic lifestyle and the absence of a Digestive System. Their habitat is the vertebrate intestine, which is rich in digestive products that serve as food for the worms; consequently, their body wall functions to absorb and process these substances.

The integuments of acanthocephalans are armed with numerous cuticular hooks derived from the Middle layer of the hypodermis. These hooks are located on the proboscis and, more rarely, on the anterior part of the trunk.

Beneath the skin-muscular sac lies a primary body cavity (schizocoel) filled with cavity fluid containing floating Cells. Through this fluid, nutrients are transported from the hypodermis to other tissues, along with Metabolic waste products.

A proboscis receptacle, extending into the body cavity, is located beneath the proboscis. It is a muscular sac whose open anterior end attaches to the walls of the proboscis, and its cavity communicates with that of the proboscis. The cavity of the proboscis receptacle houses the muscles that retract the proboscis (proboscis retractors). One end of these muscles attaches to the anterior wall of the proboscis, while the opposite end attaches to the body wall, passing through the rear wall of the receptacle. When the retractors contract, the anterior end of the proboscis is pulled into the receptacle first, followed by the entire proboscis (like turning a glove finger inside out). Additionally, the cerebral ganglion is located within the cavity of the proboscis receptacle, most commonly in its posterior part.

At the junction of the neck and trunk lie the lemnisci—specific Organs of acanthocephalans that project into the body cavity on either side of the proboscis receptacle. It is believed that lemnisci are formed by the invagination of the neck wall hypodermis into the body cavity. They have an elongated shape, resemble the body wall hypodermis in structure, and are rich in lacunae. Their exact functions remain undetermined, but most scientists lean toward the view that lemnisci participate in regulating the pressure within the proboscis receptacle during the eversion of the proboscis. Like the entire lacunar system of the hypodermis, the lemnisci likely serve as part of the transport system.

Acanthocephalans lack a digestive system; the functions of absorbing and Processing nutrients are carried out by their body wall.

The excretory system has been studied in only a few groups of acanthocephalans. It is of the protonephridial type and appears either as a capsule containing numerous flask-shaped cyrtocytes or as a treelike branched organ. The efferent duct of the protonephridium connects with the efferent ducts of the Reproductive System, forming a common opening known as the urogenital cloaca (Fig. 182).

Fig. 182. Excretory system of acanthocephalans — capsule-shaped organ (a), treelike protonephridium (b): 1 — flask-shaped cyrtocytes, 2 — nephridial capsule, 3 — excretory duct

The Nervous system of acanthocephalans is represented by a spherical or oval ganglion located inside the proboscis receptacle. Nerve branches extend forward from it to innervate the receptacle retractor muscles and proboscis receptors, while two lateral nerve trunks extend backward, giving off branches to various organs. In males, near the genital opening, there is also an additional genital nerve ganglion (Fig. 183).

Fig. 183. Diagram of the nervous system structure in the acanthocephalan Polymorphus phippsi — anterior (a) and posterior (b) ends of the male body:

1 — apical sensory organ; 2 — NERVES OF THE proboscis sheath; 3 — proboscis nerves; 4 — anterior median nerve trunk; 5 — wall of the proboscis sheath; 6 — central (main) ganglion; 7 — lateral nerves of the body wall; 8 — genital ganglion; 9 — genital nerves; 10 — sensory papillae

Sense Organs in acanthocephalans are poorly developed and represented by sensory papillae at the apex of the proboscis, lateral papillae near its base, and papillae around the genital opening. Cutaneous sensation is likewise underdeveloped in these animals; most of them show virtually no reaction to needle pricks, whereas even the slightest stimulus to the proboscis causes it to retract into its sheath.

Acanthocephalans are dioecious animals, and their reproductive system is closely associated with the so-called ligament sac. This is a thin-walled sac extending from the proboscis sheath to the posterior end of the body. Most acanthocephalans possess a single such sac, although some species have two (located near the dense axial cord).

The FEMALE REPRODUCTIVE SYSTEM has a rather peculiar structure. It consists of numerous Ovaries contained within the ligament sac. Compact ovaries are found only in young females; in sexually mature specimens, the ligament ruptures, and the ovaries break down into clusters of oocytes that float freely in the body cavity. The reproductive duct begins with a uterine bell shaped like a funnel, whose wide anterior end opens into the body cavity, while its narrowed posterior end transitions into slender oviducts and further into the Uterus. The uterus leads into the Vagina, which opens externally via the gonopore. In addition, the posterior part of the bell features another opening that also communicates with the body cavity. Eggs enter the uterine bell through the anterior opening, where a unique sorting process takes place. Mature, fertilized eggs, which have a characteristic spindle-like shape, are pushed into the narrow oviducts and subsequently forced outside through the ducts. Only fully matured eggs can pass through these narrow passages. Rounded, immature eggs and egg clumps are expelled from the uterine bell through its posterior opening back into the body cavity.

The Male Reproductive System consists of two rounded Testes located one behind the other inside the ligament sac. Vasa deferentia originate from the testes and merge into a vas deferens, the terminal section of which expands to form a Seminal Vesicle before continuing into the copulatory organ, which opens into the bursa copulatrix. In a resting state, the bursa is retracted into the male's body, but during copulation, it everts and grasps the posterior part of the female's body. Cement glands lie directly adjacent to the testes, with their ducts opening into the vas deferens. These glands secrete an adhesive substance that seals the female's genital opening after copulation, preventing the leakage of seminal fluid. Acanthocephalans undergo internal Fertilization only once in their lifetime.

Embryonic development takes place within the body of the female. By the time the eggs are released into the external environment, they contain a fully formed embryonic larva, the acanthor. For further development, the eggs must be ingested by an intermediate host, which may be a crustacean or a terrestrial insect. In the intestine of the intermediate host, the acanthor hatches from the egg, initially penetrates the gut wall, and then migrates into the body cavity. Here, a complex metamorphosis occurs, resulting in The formation of the next stage—the acanthella—and finally, the infective larva, or cystacanth. In its structure, it closely resembles the adult form, though it has an underdeveloped reproductive system. The proboscis and the posterior end are retracted into the body cavity, and the cystacanth is enclosed within a protective capsule. Infection of the definitive host occurs when it feeds on intermediate hosts whose Body Cavities contain encapsulated cystacanths (Fig. 184).

Fig. 184. Larval stages of acanthocephalans: a — acanthor; b — acanthella; c — cystacanth

The giant thorny-headed worm, Macracanthorhynchus hirudinaceus, is widely known. This largest representative of the phylum reaches a length of 650 mm. It parasitizes the intestines of domestic and wild pigs, with scarab beetle larvae serving as intermediate hosts. Because cystacanths survive insect metamorphosis, pupae and adult beetles can also serve as sources of infection (Fig. 185).

Fig. 185. Life Cycle of Macracanthorhynchus hirudinaceus:

a — adult; b — definitive host; c — egg with embryo; d — intermediate host (beetle larva); e — acanthor; f — acanthella; g — cystacanth

In many acanthocephalans, The life cycle is complicated by the inclusion of an additional host, known as a paratenic (or reservoir) host. A typical example is the cycle of acanthocephalans of the genus Corynosoma, which parasitize the intestines of marine mammals. Amphipods act as intermediate hosts, while fish serve as paratenic hosts; within the bodies of fish, the cystacanths do not develop further but simply remain viable until the fish are eaten by the definitive hosts.

Acanthocephalans parasitize humans quite rarely. Infection occurs only when a person accidentally ingests an insect or fish carrying the larval forms of acanthocephalans that normally parasitize mammals.