Exercises 4 - Platyhelminthes

• Flatworms (= platy – helminthes) are bilaterally symmetrical, dorsoventrally flattened organisms. Phylogenetically, they are the first animal phylum to possess complete organ systems.
•  The body design of the flatworm is acoelomate.
• The excretory system is made of protonephridia, which have osmoregulatory functions.
• Flatworms are usually hermaphroditic. Their sperms are biflagellated. Circulatory and respiratory systems are absent.
• Flatworms are either free-living or parasitic. Most of the approximately 30,000 species are parasitic and as much are widely distributed worldwide.
• What are the four classifications of Platyhelminthes?

  Class Turbellaria, Monogenea, Trematoda, and Cestoda
• Class Turbellaria – Free-living or commensal; most are marine, but many freshwater types can also be found; epidermis ciliated; specialized adhesive organs; mouth ventrally located.
• Class Monogenea – Mostly ectoparasitic; usually a single host; development direct (without distinct larval stages); adhesive organs present both in anterior and posterior regions; with tegument.
• Class Trematoda – Flukes. Ectoparasitic or endoparasitic; with tegument; with specialized adhesive organs; mouth anterior leading into 2-branched intestine; ovary single and testes usually paired.
• Class Trematoda

  • Subclass Digenea - Mostly endoparasitic; adhesive organs present; first host a mollusc and the final host a vertebrate. Members of this group are of medical importance to humans and domestic animals.
• Class Cestoda – Tapeworms. Endoparasitc (usually in the intestines of vertebrates); adhesive organs on scolex; body strobilate; with tegument; each segment (proglottid) bearing one or to complete hermaphroditic reproductive system; no digestive system or sense organs.
• Class Turbellaria

  Dugesia (planaria) - Planaria is a free- living flatworm that lives in freshwater ponds and creeks.
• The anterior end is marked by the paired eye spots or ocelli and ear-like flaps, called auricles that contain chemo-receptors.
• All trematodes are parasitic on the inside or outside of their hosts. They differ in having an external tegument and one or two suckers for attachment.
•  The life cycle of all trematodes involves a free- living ciliated miracidium stage that hatches from the egg.
• The first developmental stage within the snail involves the formation of sporocysts. Within the sporocysts, redia are formed which in turn, produce thousands of free-swimming cercaria.
• Fasciola hepatica (sheep liver fluke)

  PARTS:

  • Anterior sucker surrounding the mouth at the distal end.
  • Just posterior to the sucker is the pharynx that then branches off into the digestive ceca or intestine which has many branches
  • Just posterior to the intestinal bifurcation is the prominent round ventral sucker, a unique muscular structure used for attachment.
• Paragonimus (lung fluke) - Humans can be infected with this parasite by eating the metacercaria found in inadequately cooked crabs.
• Schistosoma (blood fluke) - This trematode breaks all the general rules in trematode morphology and life cycles. First, you will notice that they are dioecious worms but they are found together in pairs.
• What are the samples under class Trematoda?

  Fasciola hepatica (sheep liver fluke)

  Paragonimus (lung fluke)

  Schistosoma (blood fluke)
• All cestodes (or tapeworms) are internal parasites of invertebrates. They differ from trematodes in that they lack any type of digestive tract and they have a segmented body.
• All cestodes feed by absorbing nutrients across their tegument, and all possess an anterior holdfast on the head region called a scolex.
• What are the samples under class Cestoda?

  •  Dipylidium caninum (dog tapeworm)
  • Diphyllobothrum (-dibothriocephalus) latum (fish tapeworm)
  • Taenia solium (pork tapeworm)
  • Taenia saginata (beef tapeworm)
• Dipylidium caninum (dog tapeworm) - This tapeworm is a common parasite in the intestine of dogs and cats. The adult has about 150 proglottids.
• Diphyllobothrum (-dibothriocephalus) latum (fish tapeworm) - The scolex is almond-shaped with two elongated groove- like suckers or bothria. The scolex has no hooks. In the mature proglottid, the main organs of the female system are centrally located while the vitelline glands and testes fill the broad zones lateral to the excretory canals.
• Diphyllobothrium spp.

  Species in this genus can infect humans, particularly in cultures where eating raw fish is a cultural form.
• Hymenolepis - Members of this genus are smaller cestodes. H. nana is the dwarf tapeworm of man, while H. diminuta is parasitic in rats and mice.
• Taenia solium (pork tapeworm) - The anterior end has a knob-like scolex with 4 cup-like suckers and a round prominence, the rostellum.
• Humans can get infected with this tapeworm by eating poorly-cooked pork that is infected with the larval stage called cysticercus.
• Taenia saginata (beef tapeworm) - The mature tapeworm has more than a thousand proglottids, which are larger than that of T. solium.