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BIO2231: Phylum Platyhelminthes characteristics
BIO2231: Phylum Platyhelminthes characteristics

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Monash University

PHYLUM PLATYHELMINTHES Classification of Phylum Platyhelminthes: o No clear defining feature o In marine, freshwater, and moist terrestrial habitats o Turbellarian flatworms are mostly free-living; classes Monogenea, Trematoda, and Cestoda entirely parasitic o Bilaterial symmetry; definite polarity of anterior and posterior ends; body flattened dorsoventrally o Adult body three-layered (tripoblastic) o Body acoelomate o Epidermis may be cellular or syncytial (ciliated in some); rhadbites in epidermis of most Turbellaria; epidermis a synticial tegument in Monogenea, Trematoda, Cestoda, and some Turbellaria o Gut incomplete, may be branched, absent in cestodes o Muscular system primarily of a sheath form and of mesodermal orgin; layers of circular, longitudinal, and sometimes oblique fibers beneath the epidermis o Nervous system consisting of a pair of anterior ganglia with longitudinal nerve cords connected by transverse nerves and located in the mesenchyme in most forms o Sense organs include statocysts (organs of balance) and ocelli o Asexual reproduction by fragmentation and other methods as part of complex life cycles o Most forms monoecious; reproductive system complex, usually with well-developed gonads, ducts, and accessory organs; internal fertilization; development direct in free-swimming forms and those with single hosts; complicated life cycle often involving several hosts in many internal parasites o Excretory system of two lateral canals with branches bearing flame cells (protonephridia); lacking in some forms o Respiratory, circulatory, and skeletal systems lacking; lymph channels with free cells in some trematodes. Phylum Platyhelminthes: Platyhelminthes are divided in 4 classes: Turbellaria, Trematoda, Monogenea and Cestoda Class Turbellaria contains the free-living flatworms, along with some symbiotic and parasitic forms Most Turbellarians are bottom dwellers in marine or freshwater, living under stones or other hard objects. All members of classes Monogenea, Trematoda (flukes), and Cestoda (tapeworms) are parasitic. Most Monogenea are ectoparasites, but all trematodes and cestodes are endoparasitic. Many species have indirect life cycles with more than one host; the first host is often an invertebrate, and the final host is usually a vertebrate. Humans serve as hosts for a number of species. Form and Function – Epidermis, Muscles: Most turbellarians have a cellular, ciliated epidermis resting on a basement membrane. It contains rod-shaped rhadites, which swell and form a protective mucous sheath around the body when discharged with water Single-cell mucous glands open on the surface of the epidermis. Most orders of turbellarians have dual-gland adhesive organs in the epidermis. These organs consist of 3 cell types: viscid and releasing gland cells and anchor cells. Secretions of the viscid gland cells apparently fasten microvilli of the anchor cells to the substrate, and secretions of the releasing gland cells provide a quick, chemical detaching mechanism. In contrast to the ciliated cellular epidermis of most tubellarians, adult members of the 3 parasitic classes have a nonciliated body covering called a syncytial tegument. The term syncytial means that many nuclei are enclosed within a single cell membrane. It might appear that a completely new body covering appeared in the parasitic classes, but there are some free-living turbellarians with an atypical epidermis. Adults of all members of Trematoda, Monogenea, and Cestoda possess a syncytial covering that entirely lacks cilia and is designated a tegument. The larval forms of many of these groups are ciliated, but the ciliated coverage is shed once a host is connected. Epidermal shedding has been suggested as a means of avoiding host immune response. Development of tegument occurs as several surface layers of the epidermis are shed; eventually fused cytoplasmic extensions from cell bodies below the basement membrane become the surface covering of the body. The tegument is sometimes called the neodermis, and its shared presence among the parasites is the basis for uniting trematodes, monogeneans, and cestodes in clade Neodermata. The tegument is resistant to the immune system of the host in endoparasites, and it resists digestive juices in tapeworms and others that dwell in a host gut. The syncytial nature of the tegument might render it more resistant because there are no penetrable junctions between cells. The tegument can be both absorptive and secretory. Tapeworm tegument absorbs nutrients from the host’s digestive cavity – tapeworms have neither mouth nor gut. In the body wall below the basement membrane of flatworms are layers of muscle fibers that run circularly, longitudinally, and diagonally. A meshwork of parenchyma cells, developed from mesoderm, fills the spaces between muscles and visceral organs. Parenchyma cells in some, perhaps all, flatworms are not a separate cell type but are the non-contractile portions of muscle cells. Nutrition and Digestion: In general, Platyhelminth digestive systems include a mouth, a pharynx, and an intestine. In turbellarians, such as the planarian, the pharynx is enclosed in a pharyngeal sheath and opens posteriorly just inside the mouth, through which it can extend. The intestine has 3 many- branched trunks, one anterior and two posterior. The whole forms a gastrovascular cavity lineds with columnar epithelium. Planarians are mainly carnivorous. They can detect food from some distance by means of chemoreceptors. They entangle prey in mucous secretions from the mucous glands and rhadbites. A planarian grips prey with its anterior end, wraps its body around the prey item, extends it pharynx, and sucks up food in small amounts. Intestinal secretions contain proteolytic enzymes for extracellular digestion. Bits of food are sucked into the intestine, where phagocytic cells of the gastrodermis co
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