Phylum Platyhelminthes – Characteristics, Classification, Examples

What is Phylum Platyhelminthes?

• The Platyhelminthes, commonly known as flatworms, represent a unique group within the animal kingdom characterized by several distinctive features. These organisms exhibit a triploblastic condition, meaning their bodies are composed of three primary layers of cells: the ectoderm, mesoderm, and endoderm. This cellular structure contributes to the complexity and functionality of their bodily systems.
• One of the most notable characteristics of flatworms is their bilateral symmetry. This means that if you were to draw a line down the center of a flatworm’s body, both halves would mirror each other in terms of structure and shape. Additionally, these creatures are dorsoventrally flattened, giving them a very thin, flat appearance which aids in their movement and living conditions.
• Platyhelminthes lack several systems that are common in more complex organisms. For instance, they do not possess a circulatory or respiratory system, nor do they have a skeletal structure. Instead, their bodies are supported and their shape maintained by a soft, cellular tissue known as mesenchyme, which fills the spaces between their internal organs.
• The absence of a definitive anus is another characteristic feature of flatworms. They have a simple digestive system with a single opening that serves both as a mouth and an anus, which limits the efficiency of nutrient absorption and waste excretion. To compensate for the lack of a circulatory and respiratory system, flatworms rely on diffusion across their body surface to exchange gases and distribute nutrients.
• In terms of excretion, Platyhelminthes are equipped with a Protonephridial system. This network of tubules and flame cells helps in regulating the internal fluid and removing waste products from the body, playing a critical role in maintaining the organism’s internal environment.
• Overall, the Platyhelminthes phylum is fascinating, showcasing a significant deviation from the complex organ systems seen in higher organisms. Their simple yet effective biological structures offer valuable insights into the evolutionary adaptations that enable life in diverse environments.

Definition of Phylum Platyhelminthes

Phylum Platyhelminthes, also known as flatworms, comprises bilaterally symmetrical, triploblastic, dorsoventrally flattened invertebrates lacking circulatory, respiratory, and skeletal systems, characterized by a simple digestive system with a single opening and a protonephridial excretory system.

Characteristics of Phylum Platyhelminthes

• Habitat and Lifestyle: They can be found in various environments, leading free-living, commensal, or parasitic lives.
• Symmetry and Shape: Flatworms are bilaterally symmetrical, meaning they have symmetrical right and left sides, and dorsoventrally flattened, presenting a flattened top-to-bottom appearance.
• Cellular Structure: They are triploblastic, originating from three embryonic layers: ectoderm, mesoderm, and endoderm.
• Body Design: Their bodies are typically elongated and worm-like, with variations ranging from short, flattened forms to long, ribbon-like or leaf-like shapes.
• Size: Sizes vary widely, from microscopic to several meters in length, particularly in parasitic species.
• Segmentation: Their bodies are generally unsegmented, except in the class Cestoda (tapeworms).
• Coloration: While many are colorless or white, free-living species may exhibit a range of colors, often influenced by their diet.
• Head Differentiation: The anterior end is often differentiated into a distinct head region.
• Ventral Features: Mouth and genital openings are primarily located on the ventral (bottom) side, with variations across different groups.
• Adhesive Structures: Parasitic species possess specialized structures for attachment, such as hooks, spines, and suckers.
• Body Covering: The epidermis, which may be cellular or syncytial and often ciliated, covers their bodies. In some, like trematodes and cestodes, a cuticle replaces the epidermis.
• Body Cavity: They are acoelomates, meaning they lack a traditional body cavity, with spaces between organs filled with mesenchyme or parenchyma tissue.
• Digestive System: The digestive system, when present, is branched and incomplete, often lacking an anus. It is entirely absent in certain groups.
• Lack of Complex Systems: They do not have skeletal, respiratory, or circulatory systems.
• Excretory System: This system typically includes protonephridia with flame cells, although it may be absent in primitive forms.
• Nervous System: The nervous system is rudimentary, featuring a pair of ganglia or a simple brain and longitudinal nerve cords.
• Sensory Organs: Sensory organs are basic, more prominent in free-living species but reduced in parasitic forms.
• Reproduction: Most flatworms are hermaphrodites (monoecious) with complex reproductive systems. Asexual reproduction through fission is observed in some.
• Life Cycle: Their life cycles can be intricate, often involving multiple hosts, with mechanisms like parthenogenesis and polyembryony in some groups.

Classification of Phylum Platyhelminthes

Class 1- Turbellaria (L., turbella= a little string)

• Definition and Diversity: Turbellarians, classified under Class 1 Turbellaria, are predominantly free-living organisms, although a few adopt an ectocommensal or parasitic lifestyle. They inhabit a range of environments from terrestrial to marine and freshwater ecosystems.
• Physical Characteristics: These creatures are characterized by their unsegmented bodies, which are enveloped by a ciliated epidermis that may be cellular or syncytial in nature. This outer layer is also equipped with mucus-secreting cells and distinctive rod-shaped structures. The ventral side houses the mouth, leading to an intestine that follows a muscular pharynx.
• Adaptations: Turbellarians possess adhesive organs or suckers and are equipped with sensory organs like tangoreceptors, chemoreceptors, and photoreceptors, particularly in free-living species.
• Excretory System: The excretion process is facilitated by a network of protonephridia, with flame cells playing a crucial role.
• Reproduction: A majority engage in sexual reproduction, though asexual reproduction and regeneration are also observed. Their life cycle tends to be straightforward without complex stages.

Significant Orders within Class Turbellaria

1. Acoela Order:
   • Habitat: Exclusively marine and typically diminutive, not exceeding 2 mm in size.
   • Anatomy: These organisms lack a muscular pharynx and intestines, and their excretory system is notably absent, which includes the absence of flame cells, defined gonads, gonoducts, and yolk glands.
   • Lifestyle: Acoels are mainly free-living, dwelling under stones, in bottom mud, or amongst algae. Some species inhabit the intestines of sea urchins and sea cucumbers. Symbiotic algae may impart color to some species.
   • Acoela Suborder: Acoela lacks further subdivisions into suborders due to their relatively simple body plan and the absence of a distinct gut and excretory system.
2. Rhabdocoela Order:
   • Size and Habitat: Small organisms, generally under 3mm, found in freshwater, marine, and terrestrial settings.
   • Body Structure: Characterized by a simple pharynx and a sac-like intestine. The nervous system is structured with two main longitudinal trunks.
   • Excretory and Reproductive Systems: Possesses a protonephridia-based excretory system and a reproductive system with compact gonads and gonoducts. A cuticularized structure replaces the penis papilla, and a yolk gland may or may not be present.
   • Suborders within Rhabdocoela include:
     • Notandropora: Characterized by their exclusively freshwater habitat, simple pharynx, and a single median protonephridium for excretion. Their reproductive system includes a compact mass of testes and an unarmed penis.
     • Opisthandropora: These can be found in freshwater or marine environments. They have paired nephridia for excretion, compact testes, and an armed penis with a stylet.
     • Lecithopora: Inhabiting freshwater, marine, or terrestrial environments, this suborder is known for its bulbose pharynx and paired nephridia. They have separate ovaries and yolk glands and reproduce exclusively sexually.
     • Temnocephalida: Freshwater ectocommensals characterized by 2-12 tentacles at the anterior end and 1-2 adhesive discs at the posterior end. They have a simple reproductive system with a dolii-form pharynx.
3. Tricladida Order:
   • Size and Environment: These are larger turbellarians, ranging from 2 to 60 cm, inhabiting marine, freshwater, or terrestrial environments.
   • Anatomical Features: The mouth is located mid-ventrally, and the pharynx is usually plicate and oriented backward. The intestine divides into three branches, each with numerous diverticula.
   • Sensory and Excretory Organs: Eyes are commonly present, and the excretory system comprises lateral networks with multiple nephridiopores.
   • Suborders within Tricladida include:
     • Maricola: Exclusively marine species with a pair of eyes and auricular grooves. The copulatory apparatus is simple, sometimes equipped with a stylet.
     • Paludicola: Mostly freshwater species, occasionally brackish water dwellers. They have a variable number of eyes, and reproduction can be asexual. The bursa is typically anterior to the penis.
     • Terricola: Terrestrial species found in tropical and subtropical regions. They have elongated bodies with a variable number of eyes. Male and female antra are usually separate, and asexual reproduction is possible.
4. Polycladida Order:
   • Size and Habitat: Moderate-sized, ranging from 2 to 20 mm, primarily marine organisms that dwell on the seabed or in littoral zones.
   • Digestive and Nervous Systems: Features a plicate pharynx with a highly branched intestine. The nervous system includes numerous radially arranged nerve cords, and numerous eyes are present.
   • Reproductive Features: The reproductive system is distinguished by separate male and female gonopores, numerous testes, and ovaries scattered throughout the body.
   • Suborders within Polycladida include:
     • Acotylea: Identified by their vertical, curtain-like pharynx and absence of suckers behind the gonopore. They possess nuchal type tentacles and do not have eyes clustered at the anterior margin.
     • Cotylea: These have a tubular pharynx and a sucker behind the female pore. A pair of marginal tentacles bear eyes, or a cluster of eyes is present at the anterior margin.
5. Alloecoela Order:
   • Size and Habitat: Members of the Alloecoela order are generally moderate in size, ranging from 1 to 10 mm. They inhabit diverse environments, including marine, freshwater, and brackish water ecosystems.
   • Anatomical Features: Alloecoela turbellarians have a simple to complex pharynx, which can be bulbose or plicate in structure. Their intestines may be straight or exhibit short branches (diverticula). The excretory system is characterized by paired protonephridia, which have two or three main branches and nephridiopores for waste excretion.
   • Nervous System: These organisms possess a well-developed nervous system, typically with three or four pairs of longitudinal nerve cords connected by transverse connectives.
   • Reproductive System: The reproductive system of Alloecoela species comprises numerous testes and a pair of ovaries. A penis papilla is commonly present in these organisms, facilitating reproductive processes.
   • Lifestyle: Alloecoela turbellarians exhibit a variety of lifestyles, including free-living, ectoparasitic, and ectocommensal habits. They are versatile in their ecological roles and can be found under stones, in bottom mud, or associated with other marine organisms.
   • Suborders within the Alloecoela Order:
     • Archophora:
       • Predominantly marine organisms featuring a plicate pharynx.
       • Exhibit a more primitive female reproductive system, lacking distinct female ducts.
       • The male copulatory apparatus is relatively simple, opening towards the posterior.
     • Lecithoepitheliata:
       • Found across marine, freshwater, and terrestrial habitats.
       • Feature a simple or bulbose pharynx and a penis equipped with a cuticular stylet.
       • The female reproductive system is simplified, with minimal to no yolk glands, and the ova are surrounded by nutritive cells.
     • Cumulata:
       • Inhabit freshwater or marine environments.
       • Characterized by a bulbose or plicate pharynx and an intestine typically without diverticula.
       • The unarmed penis contrasts with the more complex reproductive systems found in other suborders.
     • Seriata:
       • Mostly found in marine and freshwater settings.
       • Distinguished by a plicate pharynx and an intestine that often features lateral diverticula.
       • The female reproductive system is notable for its separate ovaries and yolk glands, and a statocyst is usually present.

Class 2- Trematoda (Gr., trematodes= having pore)

• Nature of Parasitism: Trematodes, commonly known as flukes, are organisms that primarily engage in parasitic relationships, either as ectoparasites living on the surfaces of hosts or endoparasites residing within host organisms.
• Morphological Features: Their bodies are unsegmented, dorsoventrally flattened, and resemble a leaf in shape. The tegument, or outer covering, is thick yet lacks cilia and rhabdites, distinguishing them from other flatworms.
• Body Covering: A distinctive cuticle covers the body, providing protection but lacking division into segments.
• Attachment Organs: To adhere to their hosts, trematodes are equipped with suckers and, in some cases, hooks.
• Digestive System: Their digestive system is incomplete, featuring an anterior mouth, a simple pharynx, and a bifurcated or multiply branched intestine, with no anus.
• Nervous and Excretory Systems: They possess three pairs of longitudinal nerve cords and a protonephridial excretory system that includes flame cells for osmoregulation and waste excretion.
• Reproductive Characteristics: Trematodes are predominantly hermaphroditic, possessing a single ovary and multiple testes. Their development can be direct in ectoparasites or indirect in endoparasites, often involving an alternation of hosts.

Key Orders within Class Trematoda

1. Monogenea Order:
   • Hosts: These are mostly ectoparasites found on cold-blooded aquatic vertebrates.
   • Adhesive Structures: The anterior end typically features a pair of adhesive structures, while the posterior end is equipped with an adhesive disc, often accompanied by hooks.
   • Excretory and Reproductive Systems: Excretory pores are paired and located anteriorly. The reproductive system includes separately positioned male and female gonopores, and the uterus contains a limited number of shelled eggs.
   • Life Cycle: Monogeneans have a direct life cycle involving only one host, with a free-swimming ciliated larval stage.
2. Digenea Order:
   • Host Range: Digeneans are endoparasites that infest both vertebrates and invertebrates.
   • Suckers: They are characterized by two suckers, an oral sucker surrounding the mouth and a ventral sucker or acetabulum, both devoid of hooks.
   • Reproductive System: The reproductive tract lacks a vagina, and the uterus is typically elongated, containing numerous shelled eggs.
   • Life Cycle: Their life cycle is complex, involving multiple larval stages and one or more intermediate hosts. Larval forms undergo asexual reproduction before metamorphosis.
3. Aspidocotylea Order:
   • Suckers: Aspidocotyleans do not have oral suckers but feature large ventral suckers subdivided into multiple smaller suckers, without hooks.
   • Reproductive System: The male reproductive system contains only one testis.
   • Hosts and Habitat: They are endoparasites found primarily in the digestive systems of fishes and reptiles.

Trematoda class highlights the diversity and complexity of parasitic flatworms, with each order and suborder adapted to specific hosts and environmental conditions. Understanding the biological and ecological aspects of trematodes is crucial for managing diseases they may cause in their hosts, including humans.

Class 3- Cestoda (Gr., ketos, gridle+ eidos, form)

• Parasitic Nature: Cestodes, commonly referred to as tapeworms, are endoparasites predominantly found in the intestines of vertebrates.
• Body Structure: Their bodies are elongated, flat, and ribbon-like, divided into segments known as proglottids. In some rare instances, the body may not be segmented.
• Surface Covering: The tegument of cestodes is devoid of microvilli and is covered with a protective cuticle, lacking both epidermis and cilia.
• Adhesive Organs: The anterior end, or scolex, is equipped with adhesive structures such as hooks and suckers, aiding in attachment to the host, except in the subclass Cestodaria.
• Digestive System: Cestodes lack a mouth and a digestive system, absorbing nutrients directly through their tegument.
• Excretory and Nervous Systems: They have a protonephridial excretory system with flame cells and a nervous system typically consisting of a pair of ganglia and two lateral longitudinal nerve cords.
• Reproductive Features: Each mature proglottid is hermaphroditic, containing both male and female reproductive organs. The life cycle is complex, often involving multiple hosts, with embryos characteristically equipped with hooks.

Subclasses and Orders within Class Cestoda

Subclass 1: Cestodaria

• Habitat and Body Form: Cestodarians are endoparasitic in vertebrates, possessing an unsegmented, leaf-like body without a scolex or strobila.
• Reproduction: They have a single set of monoecious reproductive systems and a larva known as lycophore, characterized by 10 hooks.

Orders within Cestodaria:

1. Amphilinidea: Parasitic in fish coeloms, with a flattened, oval, or elongated body. Lacks a scolex and suckers, but features a protrusible pharynx and coiled uterus.
2. Gyrocotylidea: Found in fish intestines, with a distinct anterior sucker and a rosette-shaped posterior adhesive organ. Features an eversible proboscis and closely situated reproductive pores.

Subclass 2: Eucestoda

• Habitat and Structure: Eucestodes are endoparasites in vertebrate intestines, with long, ribbon-like bodies divided into a scolex, neck, and strobila consisting of multiple proglottids.
• Reproduction: Typically features several sets of monoecious reproductive organs, with larvae bearing 6 hooks.

Orders within Eucestoda:

1. Tetraphyllidea: Exclusively parasitic in elasmobranch intestines, with a scolex bearing four leaf-like bothria and spiny, armed cirri.
2. Diphyllidea: Inhabits elasmobranch intestines, with a scolex equipped with two bothria and a spiny head stalk, and a strobila with a limited number of proglottids.
3. Trypanorhyncha: Found in the spiral valve of elasmobranchs, with a scolex featuring four bothria and spiny, protrusible proboscides.
4. Pseudophyllidea: Parasitic in teleost fishes and terrestrial vertebrates, with a body that may be segmented or unsegmented. The scolex has shallow bothria, and the reproductive system includes a bilobed ovary and numerous testes.
5. Taenioidea/Cyclophyllidea: Infects the intestines of reptiles, birds, and mammals. Characterized by a large-sized tapeworm with a scolex bearing four suckers and often an apical rostellum with hooks.

Physiological Processes of Flatworms

• The species of free-living flatworms are predators or scavengers. Parasitic forms feed on their hosts’ tissues. The majority of flatworms, such as the planarian depicted in Figure 1, have a gastrovascular cavity as opposed to a complete digestive system.
• The “mouth” of these animals is also utilised to discharge waste from the digestive system. Several species have an anal entrance as well. The digestive tract might be a simple sac or greatly branching.
• Extracellular digestion occurs, with phagocytosis bringing digested components into the cells of the intestinal lining. The cestodes do not have a digestive system.
• The excretory system of flatworms consists of a network of tubules with openings to the environment and surrounding flame cells whose cilia beat to drive waste fluids accumulated in the tubules out of the body.
• The system is in charge of regulating dissolved salts and excreting nitrogenous wastes. The nervous system consists of a pair of nerve cords along the length of the body with connections between them and a big ganglion or concentration of nerves at the anterior end of the worm, where photosensory and chemosensory cells may also be concentrated.
• There is neither a circulatory nor a respiratory system; diffusion and cell-cell junctions are responsible for gas and nutrient exchange. This constrains the body thickness of these organisms, limiting them to being “flat” worms.
• Most species of flatworms are monoecious, and fertilisation normally occurs internally. Asexual reproduction is prevalent in certain populations.

Parasitic adaptations of Phylum Platyhelminthes

Taenia is an endoparasitic worm that inhabits human internal organs such as the intestine. To accommodate its parasitic lifestyle, it must overcome a number of unfavourable situations. And so, this tapeworm exhibits many modifications. The following are Taenia’s significant parasitic adaptations:

• Externally, the body is coated by tegument, which protects against the digestive activity of the host’s alkaline digestive fluids. This tegument is water and nutrient permeable.
• Within the worm, the osmotic pressure is greater than that of the surrounding host fluid. This facilitates the host’s rapid absorption of the digested meal.
• Adults and larvae lack cilia because neither need movement.
• A well-organized and developed scolex with suckers and spines is present. These suckers and spines aid in attachment, preventing the parasite from being expelled from the host’s colon as a result of peristaltic contractions.
• They inhabit places with a constant supply of pre-digested food material that is easily absorbed by the parasite. Thus, there is no need for a digestive tract. In order to improve the absorption surface area, the tegument is also changed into microvilli.
• There are no circulatory, respiratory, or sensory organs in these parasites.
• The nervous system is likewise underdeveloped since it is unnecessary.
• The reproductive system is the most developed system. An enormous quantity of eggs are produced to overcome dangers and difficulties.
• The zygotes and embryos are protected from unfavourable conditions by a protective shell.
• Hermaphroditism and proglottization ensure self-fertilization or cross-fertilization between proglottids of the same worm.

Examples of Platyhelminthes

• Dugesia (Planaria): They are found in ponds and slow-moving streams. Their body has cilia and the ability to regenerate the destroyed section. There are two eyes and two lateral lobes on the head.
• Schistosoma: It is known as blood fluke because it is discovered in the mesenteric blood vessels and hepatic portal system of humans. It exhibits considerable sexual dimorphism. Schistosoma causes Schistosomiasis, which spreads by water that has been contaminated. The patient has anaemia, discomfort, fever, enlarged liver and spleen, and diarrhoea.
• Fasciola: It is sometimes referred to as a liver fluke since it inhabits the liver and bile duct of sheep and goats. It is hermaphrodite, yet there is cross-fertilization. It causes animals to develop fascioliasis. With this condition, the animal’s liver enlarges and the bile ducts become obstructed. The infection weakens the animals’ muscles, resulting in muscular pain that may have deadly effects for the animals.
• Taenia solium: It is common in all nations where pork is consumed and is also known as the pork tapeworm. They reside as parasites in the human small intestine, and their larvae are detected in the pigs’ muscles. It is hermaphrodite and self-fertilization occurs. Taenia solium causes taeniasis, characterised by abdominal pain, anaemia, indigestion, restlessness, area, and false appetite. Other organisms such as Taenia saginata and Echinococcus granulosus are transmitted through beef in the human intestines and reside in the intestines of cats and dogs, respectively.

Significance of flatworms

• Decomposition: Flatworms serve a crucial role in decomposing organic debris, such as dead plants and animals, into compounds that can be taken by other creatures in the environment. This makes them crucial decomposers in numerous habitats.
• Food chain: Flatworms are a part of the food chain in numerous environments. They are consumed by numerous predators, including fish, birds, and insects, which facilitates the movement of energy and nutrients up the food chain.
• Research: Due of their regenerating qualities, flatworms are frequently employed in scientific studies. They are investigated in order to gain a deeper understanding of tissue regeneration and stem cell biology, which may have implications for future medicinal treatments.
• Parasitism: Although parasitic flatworms can cause harm to people and other animals, they also perform a crucial role in the ecosystem by reducing populations of other organisms that may be considered pests, such as snails.
• Bioindicators: Flatworms, which are sensitive to environmental changes such as pollution and habitat loss, can function as bioindicators of ecosystem health. Monitoring flatworm populations can provide insight into an ecosystem’s overall health.

FAQ

References

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