Platyhelminthes Lecture 3b PDF

Taxonomic Summary Phylum: Platyhelminthes Class: Turbellaria – free-living flatworms Class: Cestoda Subclass: Cestodaria Subclass: Eucestoda - the tape worms Class: Monogenea Class: Trematoda – the flukes Subclass: Digenea Subclass: Aspidogastrea P...

Taxonomic Summary Phylum: Platyhelminthes Class: Turbellaria – free-living flatworms Class: Cestoda Subclass: Cestodaria Subclass: Eucestoda - the tape worms Class: Monogenea Class: Trematoda – the flukes Subclass: Digenea Subclass: Aspidogastrea Phylum Platyhelminthes Defining Characteristics: Most species have a conspicuous anterior brain, which is connected to at least one pair of longitudinal nerve cords. The mesodermal layer of the embryo develops into a loose collection of cells known a parenchyma tissue. Most platyhelminths have no anus; food enters, and wastes leave the digestive system, through a single opening. Phylum Platyhelminthes General Characteristics: Perhaps the most conspicuous feature of platyhelminths is that they are flat. They have no specialized respiratory organs and no specialized circulatory system. Gas exchange is accomplished by simple diffusion across the body surface. Phylum Platyhelminthes General Characteristics: Metabolic wastes probably move out from flatworms mostly by diffusing across the general body surface. Most platyhelminths contain a series of specialized organs called protonephridia (first kidney). The typical protonephridium consists of a group of cilia projecting into a fine- meshed cup (Fig. 8.1). Phylum Platyhelminthes General Characteristics: The protonephridia play an important role in regulating ionic and water balance (osmoregulation) in flatworms, in addition to their possible role in eliminating metabolic wastes, including ammonia, urea, and amino acids. The vast majority of flatworm species, in all 4 classes, are simultaneous hermaphrodites. Phylum Platyhelminthes Class Turbellaria Only about 16% of all flatworm species are turbellarians. Phylum Platyhelminthes Class Turbellaria: About 150 species are commensal or parasitic with other invertebrates, but most of the 3,000 turbellarian species are free-living. Most species are marine, a number of species are found in freshwater, and a few species are considered terrestrial, although these are restricted to very humid areas. Individuals are typically less than 1 cm long. Phylum Platyhelminthes Class Turbellaria: Most aquatic turbellarian species are benthic; that is, they live in or on the ocean. Lake, pond or river bottom. The outer surface of the body is ciliated, often more so on the ventral surface than on the dorsal surface. Most species move at least partly by secreting mucus from the ventral surface and beating the ventral cilia within this viscous mucus. Phylum Platyhelminthes Class Cestoda: Defining Characteristics: 1.Small anterior hooked attachment organ (scolex) 2.Division of body into segments (proglottids) arising from anterior end, behind the scolex 3.Absence (loss) of digestive tract. Phylum Platyhelminthes Class Cestoda - Introduction Members of the class Cestoda – most of which are members of the subclass Eucestoda and commonly known as tapeworms – are all internal parasites; that is, they are endoparasitic. They are primarily parasites of vertebrates, inhabiting various regions of the host digestive tract. Phylum Platyhelminthes Class Cestoda – General Characteristics The tegument contains numerous nuclei, but these are not separated by cell membranes; that is, the tegument is syncytial (no clear membrane separation but contain lot nucleic). The outer surface is outfolded into numerous cytoplasmic projections, vastly increasing the amount of exposed surface area across which nutrients can be taken up from the Phylum Platyhelminthes Class Monogenea Defining Characteristics: 1.Posterior attachment organ (haptor (=opisthaptor)) including sucker and complex attachment hooks and sclerites; 2.Larva (oncomiracidium) bearing 3 bands of cilia and usually 1 or 2 pairs of eyes. Phylum Platyhelminthes Class Monogenea 1.Monogenetic flatworms are usually parasitic on the skin or gills of fishes; that is, most are ectoparasites. Phylum Platyhelminthes Class Monogenea The highly specialized posterior attachment organ is called the haptor (Fig. 8.15a,b). There are no intermediate hosts, so the life cycle generally involves the following stages: 1.Sexual maturity reached (in or on fish) → 2. Egg production → 3. Larval stage (oncomiracidium) → 4. Attachment to fish Phylum Platyhelminthes Class Monogenea Most monogenean species show a very high level of host specificity, and typically occupy highly specific sites within a host. Only about 1100 species have been described so far.  Phylum Platyhelminthes Class Trematoda The outer layer of adult trematode, is an unciliated, syncytial, tegument. Trematodes have a mouth opening and a blind- ended digestive tract that is, with a few exceptions, bilobed. The body is never segmented. Phylum Platyhelminthes Class Trematoda The parasite ingests the host’s tissues and blood through its mouth. Schistosomiasis, and often deadly disease prominent in many regions of the world, results from an infection by trematodes, known as ‘blood flukes’.