Lec 10: Platyhelminthes

• A coelom is a cavity that develops entirely within mesoderm.
• The body is triploblastic because it contains a middle germ layer, mesoderm, derived from endoderm.
• Acoelomate bodies do not have a coelom.
• The region between the epidermis and the digestive cavity lining is filled with a cellular, mesodermally derived parenchyma.
• Typical acoelomates have only one internal space, the digestive cavity.
• A pseudocoelomate body contains an internal cavity surrounding the gut, but this cavity is not completely lined with mesoderm, as it would be in a coelomate animal.
• Xenacoelomorpha is the sister taxon to the combined group of protostomes and deuterostomes sometimes called Bilateria or Nephrozoa
• Xenacoelomorpha is a new phylum housing two sister clades: Xenoturbellida and Acoelomorpha.
• Xenoturbellids are wormlike ciliated animals first identified in 1949.
• Xenoturbellid bodies have an external ciliated furrow, called the ring furrow, and a second longitudinal furrow called the side furrow.
• Xenoturbellids reproduce sexually; both simultaneous and sequential hermaphroditism have been described.
• Acoelomorphs are small, flat worms less than 5 mm in length.
• The word “worm” is loosely applied to elongate bilateral invertebrate animals without appendages.
• Members of Acoelomorpha were formerly placed in class Turbellaria within phylum Platyhelminthes.
• Acoelomorphs are monoecious. The female reproductive organ produces gametes and nutrition for the young at the same time; the resultant yolk-filled eggs are called endolecithal eggs.
• Members of phylum Platyhelminthes are commonly called flatworms.
• The parasitic forms of Platyhelminthes share an external body covering, called a syncytial tegument, or neodermis, which contrasts with the cellular ciliated epidermis of most free-living forms.
• Platyhelminthes is divided into four classes: Turbellaria, Trematoda, Monogenea, and Cestoda
• Class Turbellaria contains the free-living flatworms, along with some symbiotic and parasitic forms.
• All members of classes Monogenea, Trematoda (flukes), and Cestoda (tapeworms) are parasitic.
• Most Monogenea are ectoparasites, but all trematodes and cestodes are endoparasitic.
• Most turbellarians glide across surfaces using a cellular, ciliated epidermis resting on a basement membrane.
• Turbellarians contain rod-shaped rhabdites, which swell and form a protective mucous sheath around the body when discharged.
• Most orders of turbellarians have dual-gland adhesive organs in the epidermis that allow them to attach and detach from surfaces quickly.
• Dual-gland adhesive organs consist of three cell types: viscid and releasing gland cells and anchor cells.
• Secretions of the viscid-gland cells apparently fasten microvilli of the anchor cells to the substrate, and secretions of the releasing-gland cells provide a quick, chemical detaching mechanism.
• Adult members of the three parasitic classes have a nonciliated body covering called a syncytial tegument.
• The term syncytial means that many nuclei are enclosed within a single cell membrane.
• Adults of all members of Trematoda, Monogenea, and Cestoda possess a syncytial covering that entirely lacks cilia and is designated a tegument.
• The syncytial tegument is sometimes called the neodermis, and its shared presence among the parasites is the basis for uniting trematodes, monogeneans, and cestodes in clade Neodermata.
• Tapeworm tegument absorbs nutrients from the host’s digestive cavity—tapeworms have neither mouth nor gut.
• In general, platyhelminth digestive systems include a mouth, a pharynx, and an intestine.
• In turbellarians, the pharynx is enclosed in a pharyngeal sheath and opens posteriorly just inside the mouth, through which it can extend.
• Turbellarian intestine has three many-branched trunks, one anterior and two posterior.
• Planarians can detect food from some distance by means of chemoreceptors
• Planarians can entangle prey in mucous secretions from the mucous glands and rhabdites.
• Monogeneans and trematodes graze on host cells, feeding on cellular debris and body fluids.
• Cestodes have no digestive tract, they must depend on host digestion, and absorption is confined to small molecules from the host’s digestive tract
• A flame cell is cup-shaped with a tuft of flagella extending from the inner face of the cup
• Flatworms have a system of protonephridia that could be used for excretion or osmoregulation.