Identifying Characteristics of the Phylum Platyhelminthes Acoelomate Exhibit bilateral symmetry Have a bilateral nervous system with cephalization at the head end. Some species exhibit eyespots sensitive to light Possess a Gastrovascular Cavity (GVC) and primitive organ systems for digestion and excretion Do not have a circulatory or respiratory system but do take in O2 through their body surface (integumentary exchange) Hermaphroditic : can reproduce sexually (do not self fertilize) or asexually by regeneration Are motile and utilize an undulation form of motion Found in marine, freshwater and damp terrestrial habitats
Free-living Flatworms Planaria : Dugesia tigrina Freshwater, free-living flatworm Moves by beating cilia and gliding on a film of mucus 3-12mm in size Have a single opening to their stomachs in the middle of their bodies Possesses 2 eyespots (ocelli) that are sensitive to light Carnivorous (eat daphnia and midges) Common to most parts of the world Reproduce by asexual reproduction and capable of regeneration (see next slide)
Planaria Regeneration Planarians will spontaneously detach the tail end of their bodies and each half will regenerate into a full size flatworm Planarians can be cut either transversally (shown above) or dorsally and most will regenerate into a full size worm Super Cool Fact: the smallest piece of planarian to ever regenerate in a lab into a new planarian was 1/279 th of a planarian! That’s approximately 10,000 cells!
Parasitic Flatworms The pork tapeworm – Taenia solium Infects pigs and humans Lives in the intestine of its host and passes eggs through the feces Highly adapted to constant internal environments Lacks sensory organs, coordination for mobility and a digestive system (more room for reproductive structures!) Have a modified epidermis “tegument” which protects against the digestive enzymes and the immune systems of the host Can reach 7m in length in humans! Are flat and long which maximizes absorption of nutrients from the host
The tapeworm “up close and personal….” The head end has a scolex with four suckers (two seen here) that help it attach to the intestine of its host The body is separated into sections called “proglottids” that house highly developed reproductive systems (darkened areas) capable of producing hundreds of thousands of eggs and sperm
Super cool Flatworm Fact the largest tapeworm ever reported was in a sperm whale and was 30 meters in length! the largest tapeworm ever reported was in a sperm whale and was 30 meters in length! Tapeworm Scolex