LECTURE NOTES AND STUDY GUIDE IN ZOOLOGY

Module 1. ZOOLOGY AS A SCIENCE. SUBKINGDOM PROTOZOA

Content Module 1.2. Subkingdom Protozoa. General Characteristics

Topic 1.2.6. Phylum Ciliophora (Ciliates)

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 Kinetofragminophorea. Class Oligohymenophorea. Class Polyhymenophorea.

Representatives of the phylum inhabit aquatic environments; both symbiotic and parasitic forms exist. Approximately 7.5 thousand species are known. These are large organisms (50–300 µm); certain species (1–3 mm) are covered with cilia throughout their entire life cycle or only during specific phases. A characteristic feature is nuclear dualism—one large vegetative Nucleus (macronucleus) and one or more generative nuclei (micronucleus). Reproduction occurs via division or budding, and they possess a unique type of sexual process known as conjugation. The ectoplasm (cortex) features a complex Structure comprising various components that ensure the constancy of body shape. The outer part of the cortex, the pellicle, is formed by the Plasmalemma and underlying flattened vesicles interconnected in a mosaic pattern. In some species, the interior of these vesicles contains additional supporting structures—proteinaceous or calcium carbonate-impregnated polysaccharide plates. Situated beneath the pellicle are the basal bodies (kinetosomes) of all cilia. Ciliates possess three permanent structures originating from the kinetosome: 1) a transversely striated fibril located beneath the pellicle and directed toward the anterior end of The Cell; 2) a group of microtubules extending along the body toward the posterior end; and 3) a group of microtubules oriented transversely across the body. Together, these tubules form the subpellicular Cytoskeleton. The cytoskeleton—an ectoplasmic fibrillar system—is found exclusively in ciliates. Perpendicular to the pellicle surface are extrusomes, which are vesicular structures containing specialized Proteins. These structures discharge upon stimulation (appearing as elongated bodies visible under a Light Microscope) and are known as trichocysts. They are expelled outward under conditions of severe irritation, desiccation, etc., though their exact function remains incompletely understood. It is hypothesized that they contain toxic substances used to subdue enemies, or serve roles in osmoregulation and attachment. A classical object of laboratory research, the slipper animalcule (Paramecium caudatum), possesses 5,000–8,000 trichocysts. Predatory ciliates feature extrusomes known as toxocysts. These resemble capsules with a tubular envelope enclosing an inner tube. During predation, the tube everts or extends from the capsule, piercing the prey's body (other Protozoans, rotifers, etc.) and injecting venom that paralyzes or kills it. Cilia—the locomotory Organelles of ciliates—do not differ structurally from flagella. In primitive ciliates, they evenly cover the entire body surface, though they are frequently concentrated in specific areas or specialized to form cirri, membranes, or membranelles. Cirri are bundles or tufts of cilia that enable the protozoan to 'run' across a substrate or 'jump' through the Water Column. Membranes are rows of cilia fused together. In membranelles, the cilia are arranged in a row without fusing to one another, generating a water current toward the Mouth opening. The totality of all cilia and their derivatives is termed the ciliature. Cilia move in a coordinated fashion. The oar-like stroke of a cilium consists of two phases: the effective stroke and the recovery stroke. During the effective stroke, the cilium bends at its base while remaining straight, propelling fluid in the direction of movement. During the recovery stroke, it bends completely, becomes flexible, and sweeps close to the body surface with minimal water resistance, thus producing forward motion. Cilia are arranged

in rows known as kinetids (or kineses). Within each kinety, the cilia beat in the same phase, though their movement is always slightly phase-shifted relative to the neighboring kinety—a phenomenon known as metachrony. In Paramecium, rows of cilia beating in the same phase are oriented obliquely to the longitudinal body axis; under a microscope, this creates the visual effect of waves sweeping across the cell surface from back to front. This coordination is driven by the hydrodynamic Properties of the cilia, as the water current generated by one cilium synchronizes the movement phases of its neighbors.

Class Kinetofragminophorea. These are the most primitive ciliates, with bodies evenly covered in cilia. Membranelles are absent around the mouth. Cilia near the cytostome are better developed. The mouth is terminal or shifted toward the lateral body surface. A well-developed rod apparatus surrounds the cytostome. The class includes six orders.

Class Oligohymenophorea. This class comprises ciliates possessing a ciliated peristomial apparatus consisting of three membranelles on one side and a single undulating membrane on the other. This organelle complex is termed the tetrahymenium, named after the Ciliate Tetrahymena. Ciliates of this class feature an even ciliary coat or cilia restricted to the circumoral spiral. Class Oligohymenophorea includes four orders.

Class Polyhymenophorea. Ciliates in this class feature a clockwise-spiraling zone of numerous adoral membranelles. Cilia may either evenly cover the entire body or form various cirri, predominantly located on the ventral side. The class includes five orders.