Lecture Notes on Invertebrate Zoology

Semantic Module 2. Origin of Metazoans and Their Phyla: Flatworms, Roundworms, Annelids

Lecture Topic 7. Triploblastic Animals. Flatworms

Germ layer differentiation during Organogenesis. Bilateral Symmetry, increased complexity of body Structure, heightened physiological activity, and elevation of the level of Organization. The germ layer theory.

Definition of worms as a specific group of animals. Affinities of flatworms with diploblastic animals based on The structure of the digestive and nervous systems, and the capacity for regeneration. Emergence of new Organ Systems associated with The Development of three germ layers. Class Turbellaria. Structural features related to lifestyle, reproduction, and development.

Class Trematoda (digenean flukes and monogeneans). Adaptations to parasitic lifestyles. Key representatives: *Postodiplostomum*, *Hypodactyleus*, *Dactylogyrus*. Life Cycle of the Liver fluke. Methods for controlling trematodiases.

Class Cestoda — highly specialized parasites. Elaboration of adaptations to parasitism, and simplification of non-essential life-support systems as a manifestation of idioadaptations. Features of body structure, Skin-muscular sac, digestive and reproductive systems. Developmental cycles of the pork and beef tapeworms. Broad fish tapeworm. Echinococcus.

Cestodiases as dangerous diseases of humans and animals. Methods of combatting helminthiases. The Significance of K. I. Skrjabin’s works in parasitology. Social devastation caused by parasites. Significance of flatworms.

The vast majority of flatworms lead a parasitic lifestyle. Free-living flatworms occur in marine and freshwater habitats, and much less frequently on land in moist soils. The size of flatworms varies considerably. Flatworms are bilaterally symmetrical animals with a clearly defined anterior (HEAD) end. They possess distinct ventral and dorsal sides, and their bodies are typically flattened dorsoventrally, which is reflected in the name of the phylum. During embryonic development, not two, but three germ layers are established: ectoderm, endoderm, and mesoderm. The mesoderm gives rise to the musculature, Reproductive System, and undifferentiated Connective Tissue known as parenchyma (mesenchyme), which fills all the spaces between Internal Organs. A characteristic feature of flatworms is the presence of a skin-muscular sac. It consists of a single-layered integumentary epithelium, which varies in structure among different classes, and several layers of Muscles—circular, oblique or diagonal, and longitudinal. Directly beneath the skin-muscular sac lies the parenchyma, filling the intervals between the internal organs. Flatworms are classified as parenchymatous animals and lack a true body cavity. The parenchyma plays a vital role in the Organism, functioning primarily as a supportive tissue. It contains a densely developed intercellular matrix penetrated by numerous fibrils that serve a skeletal function. Parenchymal Cells are loosely arranged, leaving slit-like and lacunar spaces filled with fluid. Due to this structure, the parenchyma can act as an intermediary in The transfer of digestive products between the gut and internal organs. Concurrently, it facilitates The transport of metabolic wastes from the intercellular fluid to the excretory system. This has recently been confirmed by Electron Cell/15.html">Microscopy studies, which revealed that parenchymal cells possess non-motile projections extending into the skin-muscular sac, gut epithelium, and excretory canals, thereby establishing contact between parenchymal cells and internal organs. The parenchyma also serves as a storage site for nutrient reserves (Glycogen, Lipids, etc.). Finally, the parenchyma contains specialized mobile phagocytic cells; some of these perform a protective function by engulfing Bacteria and foreign particles, while others have an excretory role by accumulating solid wastes. The Digestive System in many primitive free-living species is incomplete, and in some parasitic forms, it is completely reduced. It consists of a Mouth, an ectodermal foregut (Pharynx), and an endodermal blind-ending midgut. Often, the midgut is branched, with its diverticula permeating the entire body. Thus, this division of the digestive system ensures not only food Digestion but also its transport to all PARTS OF THE body. Flatworms are the first group to develop a specialized protonephridial-type excretory system. Most groups possess two branching, tree-like protonephridial canals opening to the outside through either two separate pores or a single common pore. The inner terminal ends of the canals terminate in specialized flame cells, also known as cyrtocytes or ciliated cells. Flatworms lack specialized respiratory organs, as well as a Circulatory system. The Nervous system varies in structure, but in most representatives, it is of the orthogenous (ladder-like) type: longitudinal nerve cords extend from a cerebral ganglion located at the anterior end of the body and are interconnected by transverse bridges called commissures. Sense Organs are predominantly represented by epidermal sensilla containing sensory Nerve Cells with one or more cilia. These sensilla perceive mechanical and chemical stimuli. Some flatworms possess eyespots and statocysts, which are organs of balance. With few exceptions, flatworms are hermaphrodites. Primitive representatives of this phylum (lower turbellarians) lack well-defined Gonads, with Germ Cells scattered throughout the parenchyma; Fertilization is internal. More highly evolved forms possess true gonads—Ovaries and Testes—ducts for the elimination of sperm and mature eggs, as well as an array of accessory organs ensuring internal fertilization, the storage of allosperin, eggshell formation, etc. The structural details of the reproductive system vary widely among different classes. In most free-living turbellarians, development is direct; the hatching juvenile differs from the adult only in size and an undeveloped reproductive system. Some marine polyclads develop through metamorphosis, featuring a planktonic Müller's larva. In parasitic forms, The life cycle is typically very complex, involving several generations with different structures. One generation reproduces sexually, while the next reproduces parthenogenetically (from unfertilized eggs). This Morphology/12.html">ALTERNATION OF GENERATIONS is termed heterogony. Alongside sexual reproduction, asexual reproduction occurs in certain groups. The phylum Platyhelminthes comprises nine classes, of which three are the most widespread: 1) Turbellarians or flatworms (Turbellaria), 2) Flukes or trematodes (Trematoda), and 3) Tapeworms or cestodes (Cestoda). Tricladids are an order of turbellarians, predominantly large animals (up to 50 cm in length) inhabiting marine, freshwater, and terrestrial soils. In various European Water bodies, one can easily find the milk-white planarian (*Dendrocoelum lacteum*) and the black multi-eyed planarian (*Polycelis nigra*). Planarians feed on oligochaetes, small Mollusks, and Arthropods. All trematodes are endoparasites. Adults (maritae) inhabit primarily various sections of the digestive tract of vertebrates, as well as their Lungs, Kidneys, body cavity, and circulatory system. Many of them are causative agents of severe diseases in humans and agricultural livestock. Trematode sizes range from 0.3–0.4 mm to 3–7.6 cm. Over 4,000 species are known, with about 600 species recorded in Ukraine. The class Cestoda encompasses approximately 3,500 species of endoparasites (over 500 species known in Ukraine) that parasitize vertebrates during their sexually mature stage, whereas their larval stages typically inhabit invertebrates, particularly arthropods. In some species, larvae also parasitize vertebrates, including humans.