Platyhelminthes

1. Platyhelminthes Chris Racz

2. Examples Turbellaria: Free living flatworms, Planarians, Symsagittifera roscoffensis, Microstomum caudatum. Cestoda: Cyclophyllidea, Pseudophyllidea, Taenia saginata, Diphyllobothrium, Taenia. Trematoda: Digenea, Aspidogastrea, tissue flukes, blood flukes Monogenea: Polyopisthocotylea, Monopisthocotylea, Gyrodactylus, Dactylogyrus, Neobenedenia, earthworms

3. Characteristics • Bilaterally symmetrical • Body is made of 3 layers of tissues with organs/organelles (tripoblastic) • No internal cavities • No anus, classified as a blind ending gut • Regurgitates through mouth using protonephredia • Nervous system containing longitudinal fibers instead of a net • Flat because they lack circulatory and respiratory organs • Hermaphrodite sexual reproduction • Cestoda, Trematoda, and Monogenea are parasites; Turbellaria are non-parasitic. • Soft bodied • Hermaphroditic • Have neoblasts---stem cells that give rise to the other cell types in the body • Made of epidermal cells that are typically multiciliated, each cell bearing multiple cilia rather than only one • Acoelomates

4. Morphology: Basic Structure The structure of platyhelminthes are bilaterally symmetric, containing three cell layers. They have definitive nervous tissues and sensory organs at one end of their body, distinctly showing the head and tail. They lack circulatory and respiratory organs and do not have an internal body cavity. It has two eye-slots on its head, and body is covered in ciliated epidermis.

5. Morphology: Systems Digestive system consists of the mouth, pharynx, and a branching intestine where the nutrients are absorbed. The excretory system has unique flame cells, which help excrete waste through their body. When cilia beat, they move water into the tubules and out pores in the body called nephridiospores. The muscle system, made of muscle fibers, is below the epidermis and is used for locomotion. The nervous system is made of longitudinal nerve cords and smaller lateral nerves.

6. Embryology Like most animals, platyhelminthes have 3 germ layers to protect the embryo: the ectoderm, endoderm and mesoderm. The mesoderm is the embryonic layer that forms in between the ectoderm and endoderm. It has no true body cavity. Turbellaria are hermaphrodites, fertilizing their eggs internally by copulation. Larger ones mate by penis fencing, a duel trying to impregnate the other. the fluke has a long coiled tube, or uterus, that stores eggs until release through a genital pore. A fluke may produce tens of thousands of eggs at a time. A few survive predation and environmental destruction and hatch into larvae, or immature organisms. Cestoda contains both male and female reproductive organs. Cross-fertilization between two adjacent worms is typical, but self-fertilization between proglottids can also occur. After fertilization the egg-packed proglottid breaks off from the adult and is eliminated with the host’s feces. Monogenea form a mucus coat around part of their intertwined bodies. Each injects sperm into the mucus. Sperm from one worm moves to the pouchlike seminal receptacle of the other. Simultaneously eggs in the body cavity move through oviducts to the female genital pore. After several days a mucus and chitin sheath is secreted by the clitellum, a swelling around the sex organs. As the worm wriggles to slip the sheath off its body, eggs and sperm are joined and fertilization occurs.

7. Biochemical Evidence James V. McConnell discovered that flatworms had memory RNA when he cut them in half. After cutting them in half, they regenerated into two full flatworms

8. Life History of a blood fluke A female fluke fits into a groove inside of the males body. Once inserted, the blood flukes reproduce sexually in the human host. Once fertilization occurs, the eggs exit the hosts feces. The egg develops in water to form ciliated larvae, which infect snails as the intermediate host. Asexual reproduction within the snail results in another motile larva, which escapes the host. The larvae penetrate the skin and blood vessels of human inside water contaminated with the infected human feces.

9. Summary Platyhelminthes are flatworms with bilateral symmetric body. They are a very interesting specimen due to memory RNA, capable of reforming after being sliced in half. Platyhelminthes consists of the unsegmented flatworms, which includes both free-living and parasitic species. They have bilateral symmetry, and can move by using layers of muscles, or in some species, by gliding along a slime trail using cilia. Flatworms are slightly less developed than segmented worms due to their lack of a circulatory system and complete digestive system. Instead, flatworms absorb nutrients through their skin and excrete wastes using specialized "flame cells." Some flatworms have primitive light-sensing "eyes" that allow them to move either towards or away from light, while other species have different types of sensors on their bodies, including chemical, balance, and water movement receptors. Most species of flatworms reproduce either sexually or asexually.