SUPPORTING LECTURE NOTES IN ZOOLOGY

Module 2. SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA). DIVISION LOWER METAZOA (PROMETAZOA). EUMETAZOA

Content Module 2.3. Flatworms, Nemerteans, Rotifers, Thorny-Headed Worms

Topic 2.3.4. Phylum Thorny-Headed Worms (Acanthocephala)

General characteristics of the phylum. Origin, evolution, paleontological records, modern diversity, geographical distribution, and ecological and medical/economic significance. A list of key taxonomic groups important for human life and the functioning of natural ecosystems. Class Acanthocephala.

Parasitic worms comprising about 1,000 species, with approximately 60 species found in Ukraine. Adults parasitize the intestines of vertebrates, whereas larvae inhabit the body cavity of Arthropods. Bilaterally symmetrical animals featuring an attachment proboscis armed with cuticular hooks at the anterior end. Ciliated covering is completely reduced; the body is covered by a cuticle. They possess a Skin-muscular sac and a primary body cavity, the schizocoel. The Digestive System is entirely reduced, and the excretory system is of the protonephridial type. The Nervous system consists of a cerebral ganglion (endon) and a pair of lateral nerve cords. Gonochoristic (dioecious), with a urogenital cloaca. Eggs undergo spiral Cleavage, development proceeds with metamorphosis. The life cycle involves alternation of hosts. The phylum Acanthocephala includes a single class — Acanthocephala.

Class Thorny-Headed Worms (Acanthocephala). Body size ranges from 1.5 to 650 mm. Coloration is whitish, occasionally bright orange or brown. The body is elongated and cylindrical, consisting of

a proboscis, a neck, and a trunk. The proboscis serves as the primary attachment organ. In various species, it may be cylindrical, oval, or globular in shape. Cuticular hooks are arranged in longitudinal or spiral rows along its entire length. These hooks have a characteristic Structure 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 function on the anchor principle. The lower part of the proboscis and the neck are devoid of hooks. The skin-muscular sac consists of an epicuticle, a cuticle, and a hypodermis underlaid by a basement membrane and two Muscle layers (circular and longitudinal). The epicuticle is a thin film containing mucopolysaccharides, which protects the parasite from the host's digestive Enzymes. The cuticle is a thin layer externally covered by a Cell/33.html">Plasma Membrane and pierced by numerous pores, from which canaliculi extend into the hypodermis (serving protective and absorptive Functions). Nutrient molecules enter the hypodermis through pore canaliculi, where they are digested. The hypodermis has a syncytial structure. It is the thickest layer of the integument, which is further subdivided into three layers differing in structure and the arrangement of supporting fibers. Fibers are most abundant in the middle layer, where they intertwine, providing structural support for the integument. The lower layer features a complex lacunar system resembling clefts between bundles of radial fibers filled with fluid. This layer houses the nuclei of the hypodermal syncytium and the highest concentration of Mitochondria, where active metabolic processes take place. The lacunar System of the hypodermis is connected with tubular Muscles, fulfilling a transport and metabolic function. The lower layer of the hypodermis contains large amounts of Glycogen as a reserve substance.