COMPREHENSIVE LECTURE NOTES IN ZOOLOGY

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

Content Module 2.3. Flatworms, Nemerteans, Rotifers, Spiny-headed Worms

Topic 2.3.1. Phylum Flatworms (Plathelminthes)

General characteristics of the phylum. Origin, evolution, paleontological data, modern diversity, geographical distribution, and Significance in nature and human life. Overview of major taxonomic groups important for human life and the functioning of natural ecosystems. Class Turbellaria. Class Xenoturbellida. Class Gnathostomulida. Class Trematoda. Class Aspidogastrea. Class Monogenoidea. Class Gyrocotylida. Class Cestoda. Class Amphilinoidea.

Most flatworms are parasites. Free-living flatworms inhabit marine and freshwater environments, and much less frequently, terrestrial damp soils. Their size ranges from 0.5 mm to 30 m (in whale gut parasites). Approximately 12,000 species are known. They are bilaterally symmetrical animals with a well-defined anterior end. Ventral and dorsal sides are distinguishable, and the body is flattened dorsoventrally. During embryonic development, three germ layers are formed: ectoderm, endoderm, and mesoderm. The mesoderm gives rise to Muscles, the Reproductive System, and undifferentiated Connective Tissue—the parenchyma (mesenchyme)—which fills the spaces between Internal Organs. The integumentary-muscular sac consists of a single-layered surface epithelium of varying Structure and several Muscle layers: circular, oblique or diagonal, and longitudinal. The vermiform (worm-like) movement of flatworms is driven by the contraction of various muscle groups within this sac. Bundles of dorsoventral muscles penetrate the parenchyma. There are also muscles associated with the Pharynx, attachment organs, reproductive system, etc. The parenchyma lies beneath the integumentary-muscular sac. Flatworms belong to parenchymatous animals and lack a true body cavity. The parenchyma serves as a supporting tissue. It features an extensively developed Extracellular matrix penetrated by numerous fibrils (supportive structures). Parenchyma Cells are loosely arranged, with slit-like and lacunar fluid-filled spaces between them. The parenchyma can act as an intermediary in The transport of digestive products between the gut and internal organs. Simultaneously, it facilitates the transport of metabolic wastes from the intercellular fluid to the excretory system. Parenchyma cells possess non-motile projections that extend into the integumentary-muscular sac, intestinal epithelium, and excretory canals, ensuring contact between parenchymatous cells and internal organs. The parenchyma serves as a storage site for nutrients (Glycogen, Lipids, etc.). It also contains specialized mobile phagocytic cells; some of these perform a protective function by engulfing Bacteria and foreign particles, while others perform an excretory function by accumulating solid wastes.

Class Turbellaria. Free-living predators inhabiting aquatic environments and occasionally damp soils. Some turbellarians have transitioned to various symbiotic relationships with Echinoderms, crustaceans, sipunculans, Annelids, Arthropods, and fish, and true parasites also exist. About 3,000 species are known. Ukrainian freshwaters host approximately 100 species of turbellarians, with roughly the same number found in the Black and Azov Seas. The body shape is leaf-like, ribbon-like, or spindle-like. Most lack any appendages, though some develop tentacle-like outgrowths on the anterior end. Body size typically does not exceed 1 cm, large forms reach 5–6 cm, and terrestrial tropical turbellarians can grow up to 60 cm (Bipalium). The bodies of most turbellarians are brightly colored.

Class Xenoturbellida. Represented by a single species, Xenoturbella bocki, found in marine environments at depths of 40–100 m on muddy bottoms. Size ranges from 2 to 3 cm. The excretory system and cerebral ganglion are absent. The Nervous System is a nerve plexus located within the basal region of the covering epithelium. A pair of ciliated grooves is present on the sides of the anterior end, with a circular ciliated groove located posterior to the Mouth. These structures presumably function as Sensory Organs. The Development of xenoturbellids remains unstudied.

Class Gnathostomulida. Over 80 species are known. These are marine animals ranging in size from 0.3 to 0.5 mm. The body is elongated, with distinct HEAD and tail regions. The covering epithelium is ciliated, with each Cell bearing a single cilium. The mouth opening is located near the anterior end of the body. In front of the pharynx, There is a sclerotized plate used by the animals to scrape food—such as unicellular blue-green Algae, fungal hyphae fragments, and bacteria—from substrates. The pharynx leads into a blind, sac-like midgut.

Class Trematoda, or Digeneans (Trematoda, or Digenea). Endoparasites. Adults (maritae) primarily inhabit various sections of the digestive tract of vertebrates, as well as their Lungs, Kidneys, body cavity, and Circulatory system. Many are causative agents of severe diseases in humans and livestock. Sizes range from 0.3–0.4 mm to 3–7.6 cm. Over 4,000 species are known, with about 600 species found in Ukraine. The body shape is leaf-like. Adult worms possess two well-developed suckers. One (the oral sucker) is located at the anterior end and contains the mouth opening in its center, while the other (the ventral sucker) Functions solely as an attachment organ (situated in the middle of the body, though displaced closer to the anterior or posterior end in some species). The degree of sucker development depends on the parasite's localization site. Trematodes inhabiting the intestinal tract have robust suckers, whereas those dwelling in the body cavity or Blood Vessels have underdeveloped suckers or lack them entirely.

Class Monogenoidea. Parasitic forms, predominantly ectoparasites of fish. Endoparasitic species are also known, living in the Oral Cavity, hindgut, or Ureters of fish, and the urinary bladders of turtles and frogs. A single species, Oculotrema hippopotami, parasitizes beneath the eyelids of hippopotamuses. About 600 species are known, with roughly 200 in Ukraine. These are small animals (ranging from fractions of a millimeter to 3 cm), with marine forms generally larger than freshwater ones. The body is elongated and dorsoventrally flattened. It is divided into two regions: the body proper (trunk) and the attachment apparatus. The anterior end features attachment organs of several types. In more primitive forms, these appear as cephalic outgrowths—one or two pairs of highly mobile lobes containing ducts of unicellular glands that secrete a sticky substance. The most advanced organs are attachment pads, which are two symmetrical thickenings on the sides of the head where the ducts of head glands also open. In some species, these pads transform into head pits capable of functioning not only by adhesion but also on THE PRINCIPLE OF a pneumatic sucker. Additionally, a muscular pad, or true sucker, is formed around the mouth opening.

Class Cestoda. Approximately 3.5 thousand species of endoparasites (over 500 species are known in Ukraine) that inhabit vertebrates as adults (with the exception of Representatives of the genus Archigetes, which live in the body cavity of oligochaetes) and invertebrates, particularly arthropods, during their larval stage. In some species, larvae also parasitize vertebrates, including humans. The body is divided into a scolex (head), followed by an unsegmented neck ("growth zone"), behind which segments (proglottids) bud off to form the strobila. Some cestodes are unsegmented (monozoic), while others possess two to four or thousands of segments. Sizes range from a few millimeters to 10 m. Giants exceeding 20 m are known among members of the family Diphyllobothriidae. The scolex bears attachment organs. The shape of the head is rounded, elongated, or flattened. Attachment organs are diverse, taking the form of sucker-like bothria or complex bothridia formed by elongated suckers divided by numerous septa. Most commonly, the scolex bears four, sometimes one or five, powerful cup-shaped suckers, and some species additionally feature hooks of various structures located on the head or on four long probosces (genus Tetrarhynchus). In certain cestodes, the anterior part of the body enlarges to form a folded rosette, which the parasite uses to anchor itself to the host's intestinal wall.