Platyhelminthes
Introduction:
Platyhelminthes consists of the soft bodied flukes and flatworms which have a range of different life-histories and forms ranging from free-living, commensal (relying on a host for survival without damage to the host) or parasitic.
Their distinguishing feature is their flat body due to the lack of a body cavity with only one opening used both for bringing in food and expelling waste. Unlike other organisms they lack specialised systems, possessing a simple nervous system but no circulatory or respiratory system. Most are hermaphrodites, containing both male and female organs- as a result many are capable of self-fertilisation.
All platyhelminthes are either predators or scavengers, many are parasites feeding on the tissue of the host. As a result these individuals lack a digestive system and various organs, relying completely on the host for the acquisition of nutrients and disposal of waste. Often they can live in the host's body for several decades.
In the museum platyhelminthes can be found in outer cabinet 4 along with the nematodes
Adaptations:
Flatworms are versatile animals built for survival in hostile environments. They have well developed muscles for moving, with some relying on cilia for crawling or swimming. They are well known for their incredible regenerative ability due to their neuroblast cells- specialist stem cells which can turn into any cell type. If you cut a flatworm in half two worms will grow.
The flattened shape of the Platyhelminthes allows it to survive without circulation as even the innermost cells are still close enough to the surface of the animal to receive oxygen and remove waste via diffusion.
Parasitic Platyhelminthes are equipped with a range of adaptations to suit their unique lifestyle. These include:
Loss of unwanted organs (may loose gut as directly absorbing materials from host)
Penetration devices (get into host. Often enzymes/ tail)
Attachment devices (attach inside/ on top of hosts. Suckers/hooks)
Protective devices (protective coverings, mucous, some way to escape immune system/ resist host enzymes).
Transmission via a vector (another animal that isn't a host but will help transport the parasite)
Production of eggs in large numbers (ensure they get passed on).
Classification:
The platyhelminthes are classified based on their life-history (if they are free living vs parasitic) and their habitat.
Turbellaria
Free-living, usually marine.
Named after the turbulence their beating cilia create in the water as they swim.
Monogenea
Small ectoparasite flatworms often found on the skin or gills of fish (ectoparasites)
Trematoda
Flukes- internal parasites of a range of species, including humans
Cestoda
Tapeworms - internal parasites of a number of vertebrate species
Explore the different Platyhelminthes groups with this interactive phylogeny tree from OneZoom
Museum Highlights:
Dissected fish showing tapeworm inside- likley the cause of death
Giant liver fluke which can grow up to 8cm long and 3cm wide. These infect multiple hosts throughout their life cycle, relying on snails as intermediate hosts to pass them on to deer, elk and caribou, their definitive host.
Tapeworm removed from human intestines
modification of work by Sripa B, Kaewkes S, Sithithaworn P, Mairiang E, Laha T, and Smout M; credit b: modification of work by Centers for Disease Control and Prevention from https://bio.libretexts.org/Courses/Portland_Community_College/Cascade_Microbiology/05%3A_The_Eukaryotes_of_Microbiology/5.2%3A_Parasitic_Helminths Turbellarian: By Richard Ling - Flickr, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=1887311 Monognea: Spatio-temporal variation and the use of host body surface by ectoparasites of the chelonians Phrynops geoffroanus and Mesoclemmys tuberculata in areas of the Caatinga and Atlantic Forest in northeast Brazil - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Monogenea-Polystomatidae-Polystomoides-brasiliensis-whole-specimen-of-freshwater_fig3_330985732 [accessed 24 Nov, 2023] Trematoda: M.B. Rokni, in Encyclopedia of Food Safety, 2014 Cestoidea: By http://phil.cdc.gov/PHIL_Images/20031208/87d4bff74e41427cb278526bd9cbe76a/5260_lores.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=776561