PHYLUM PLATYHELMINTHES
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 manybranched 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 complete digestion (intracellular). Undigested food is egested through the pharynx. Monogeneans and trematodes graze on host cells, feeding on cellular debris and body fluids. The mouth of trematodes and monogeneans usually opens at or near the anterior end of their body into a muscular, nonextensible pharynx. Posteriorly, their oesophagus opens into a blindly ending intestine, which is commonly Y-shaped but may be highly branched or unbranched. Because cestodes have no digestive tract, they must depend on host digestion, and absorption is confined to small molecules from the host’s digestive tract. Excretion and Osmoregulation: Excretory systems remove wastes from the body, whereas osmoregulatory systems control water balance. Osmoregulatory systems are very commin in freshwater animals where concentration gradients between internal fluids and the external environment cause bloating as water enters across the body’s permeable membranes. Excess water is typically removed via osmoregulatory systems. Sometimes osmoregulation and excretion are combined when waste products are dissolved in water shed from the body. Flatworms have system of protonephridia that could be used for excretion or osmoregulation. Although a small amount of ammonia is excreted via protonephridia, most metabolic wastes are largely removed by diffusion across the body wall.
Flatworm protonephridia (excretory or osmoregulatory organs closed at the inner end) have flame cells. A flame cell is cup-shaped with a tuft of flagella extending from the inner face of the cup. In some turbellarians and in all Neodermata, the protonephridia form a weir, the rim of the cup is elongated into fingerlike projections that extend between similar projections of a tubule cell. The space (lumen) enclosed by the tubule cell continues into collecting ducts that finally open to the outside by pores. Beating flagella drive fluid down the collecting ducts and provide a negative pressure to draw fluid through the delicate interlacing projections of the weir. The wall of the duct beyond the flame cell commonly bears folds or microvilli that probably function in resorption of certain ions or molecules. In planarians, collecting ducts join and rejoin into a network along each side of the animal and may empty through many nephridiopores. This system is mainly osmoregulatory because it is reduced or absent in marine turbellarians, which do not have to expel excess water. Flame cell protonephridia are present also in the parasitic taxa. Monogeneans usually have two excretory pores opening laterally, near the anterior. Collecting ducts of trematodes empty into an excretory bladder that opens to the exterior by a terminal pore. In cestodes, there are two main excretory canals on each side that are continuous through the entire length of the worm. They join in the last segment to form an excretory bladder that opens by a terminal pore. When the terminal proglottid is shed, the two canals open separately. Nervous System: The most primitive flatworm nervous system, found in some turbellarians, is a subepidermal nerve plexus resembling the nerve net of cnidarians. Other flatworms have, in addition to a nerve plexus, one to five pairs of longitudinal nerve cords lying under the muscle layer. Freshwater planarians have one ventral pair. Connecting nerves form a ‘ladder-type’ pattern. Their brain is a bilobed mass of ganglion cells arising anteriorly from the ventral nerve cords. Neurons are organized into sensory, motor, and association types. Sense Organs: Active locomotion in flatworms has favored not only cephalization in the nervous system. Ocelli, or light-sensitive eyespots, are common in turbellarians, monogeneans, and larval trematodes. Some species also have statocysts for equilibrium and rheoreceptors for sensing direction of the water current. Sensory endings are abundant around the oral sucker of trematodes and holdfast organ (scolex) of cestodes and around genital pores in both groups.
Reproduction and Regeneration: Many turbellarians reproduce both asexually (by fission) and sexually. Asexually, freshwater planarians merely constrict behind the pharynx and separate into two animals, each of which regenerates the missing parts-a quick means of population increase. Reduced population density may cause an increase in the rate of fissioning. In some fissioning forms individuals may remain temporarily attached, forming chains of zooids. Trematodes undergo asexual reproduction in their intermediate hosts, snails. Classification of Phylum Platyhelminthes: Class Turbellaria: Usually free-living forms with soft, flattened bodies; covered with ciliated epidermis containing secreting cells and rodlike bodies (rhabdites); mouth usually on ventral surface sometimes near center of body; no body cavity except intercellular lacunae in parenchyma; mostly hermaphroditic, but some have asexual fission. Class Trematoda: Body of adults covered with a syncytial tegument without cilia; leaflike or cylindrical in shape; usually with oral and ventral suckers, no hooks; alimentary canal usually with two main branches; mostly monoecious; development indirect, with first host a mollusk, final host usually a vertebrate; parasitic in all classes of vertebrates Class Monogenea: Body of adults covered with a syncytial tegument without a cilia; body usually leaflike to cylindrical in shape; posterior attachment organ with hooks, suckers, or clamps, usually in combination; monoecious; development direct, with single host and usually with free-swimming, ciliated larva; all parasitic, mostly on skin or gills of fish Class Cestoda: Body of adults covered with nonciliated, syncytial tegument; general form of body tapelike; scolex with suckers or hooks, sometimes both, for attachment; body usually divided into series of proglottids; no digestive organs; usually monoecious; larva with hooks; parasitic in digestive tract of all classes of vertebrates; development indirect with two or more hosts; first host may be vertebrate or invertebrates
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 manybranched 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 complete digestion (intracellular). Undigested food is egested through the pharynx. Monogeneans and trematodes graze on host cells, feeding on cellular debris and body fluids. The mouth of trematodes and monogeneans usually opens at or near the anterior end of their body into a muscular, nonextensible pharynx. Posteriorly, their oesophagus opens into a blindly ending intestine, which is commonly Y-shaped but may be highly branched or unbranched. Because cestodes have no digestive tract, they must depend on host digestion, and absorption is confined to small molecules from the host’s digestive tract. Excretion and Osmoregulation: Excretory systems remove wastes from the body, whereas osmoregulatory systems control water balance. Osmoregulatory systems are very commin in freshwater animals where concentration gradients between internal fluids and the external environment cause bloating as water enters across the body’s permeable membranes. Excess water is typically removed via osmoregulatory systems. Sometimes osmoregulation and excretion are combined when waste products are dissolved in water shed from the body. Flatworms have system of protonephridia that could be used for excretion or osmoregulation. Although a small amount of ammonia is excreted via protonephridia, most metabolic wastes are largely removed by diffusion across the body wall.
Flatworm protonephridia (excretory or osmoregulatory organs closed at the inner end) have flame cells. A flame cell is cup-shaped with a tuft of flagella extending from the inner face of the cup. In some turbellarians and in all Neodermata, the protonephridia form a weir, the rim of the cup is elongated into fingerlike projections that extend between similar projections of a tubule cell. The space (lumen) enclosed by the tubule cell continues into collecting ducts that finally open to the outside by pores. Beating flagella drive fluid down the collecting ducts and provide a negative pressure to draw fluid through the delicate interlacing projections of the weir. The wall of the duct beyond the flame cell commonly bears folds or microvilli that probably function in resorption of certain ions or molecules. In planarians, collecting ducts join and rejoin into a network along each side of the animal and may empty through many nephridiopores. This system is mainly osmoregulatory because it is reduced or absent in marine turbellarians, which do not have to expel excess water. Flame cell protonephridia are present also in the parasitic taxa. Monogeneans usually have two excretory pores opening laterally, near the anterior. Collecting ducts of trematodes empty into an excretory bladder that opens to the exterior by a terminal pore. In cestodes, there are two main excretory canals on each side that are continuous through the entire length of the worm. They join in the last segment to form an excretory bladder that opens by a terminal pore. When the terminal proglottid is shed, the two canals open separately. Nervous System: The most primitive flatworm nervous system, found in some turbellarians, is a subepidermal nerve plexus resembling the nerve net of cnidarians. Other flatworms have, in addition to a nerve plexus, one to five pairs of longitudinal nerve cords lying under the muscle layer. Freshwater planarians have one ventral pair. Connecting nerves form a ‘ladder-type’ pattern. Their brain is a bilobed mass of ganglion cells arising anteriorly from the ventral nerve cords. Neurons are organized into sensory, motor, and association types. Sense Organs: Active locomotion in flatworms has favored not only cephalization in the nervous system. Ocelli, or light-sensitive eyespots, are common in turbellarians, monogeneans, and larval trematodes. Some species also have statocysts for equilibrium and rheoreceptors for sensing direction of the water current. Sensory endings are abundant around the oral sucker of trematodes and holdfast organ (scolex) of cestodes and around genital pores in both groups.
Reproduction and Regeneration: Many turbellarians reproduce both asexually (by fission) and sexually. Asexually, freshwater planarians merely constrict behind the pharynx and separate into two animals, each of which regenerates the missing parts-a quick means of population increase. Reduced population density may cause an increase in the rate of fissioning. In some fissioning forms individuals may remain temporarily attached, forming chains of zooids. Trematodes undergo asexual reproduction in their intermediate hosts, snails. Classification of Phylum Platyhelminthes: Class Turbellaria: Usually free-living forms with soft, flattened bodies; covered with ciliated epidermis containing secreting cells and rodlike bodies (rhabdites); mouth usually on ventral surface sometimes near center of body; no body cavity except intercellular lacunae in parenchyma; mostly hermaphroditic, but some have asexual fission. Class Trematoda: Body of adults covered with a syncytial tegument without cilia; leaflike or cylindrical in shape; usually with oral and ventral suckers, no hooks; alimentary canal usually with two main branches; mostly monoecious; development indirect, with first host a mollusk, final host usually a vertebrate; parasitic in all classes of vertebrates Class Monogenea: Body of adults covered with a syncytial tegument without a cilia; body usually leaflike to cylindrical in shape; posterior attachment organ with hooks, suckers, or clamps, usually in combination; monoecious; development direct, with single host and usually with free-swimming, ciliated larva; all parasitic, mostly on skin or gills of fish Class Cestoda: Body of adults covered with nonciliated, syncytial tegument; general form of body tapelike; scolex with suckers or hooks, sometimes both, for attachment; body usually divided into series of proglottids; no digestive organs; usually monoecious; larva with hooks; parasitic in digestive tract of all classes of vertebrates; development indirect with two or more hosts; first host may be vertebrate or invertebrates
