Chapter 13 - Introduction to Phylum Platyhelminthes General Characteristics • Bilaterally symmetrical; dorsoventrally flattened • Acoelomates; most of the body is comprised of parenchyma, a collection of fibers and cell types (sensory, food storage, etc.); internal organs are embedded in the parenchyma; muscle fibers are also found running through the parenchyma • Unable to synthesize fatty acids and sterols de novo • Turbellaria and free-living stages (certain larval stages) of Class Cestoidea and Trematoda have ciliated epithelium General Characteristics cont. • The nervous system does have a centralized (ganglionated) area and a peripheral system (arranged as a network or as a ladderlike system with longitudinal nerve cords and transverse commissures) • The digestive system is typically a blind sac and most flatworms have a mouth near the anterior end followed by a muscular pumping pharynx General Characteristics cont. •Anus absent; wastes are eliminated through the mouth or via nephridiopores • In some the gut is a simple sac, while in others it is finely branched structure; some groups (cestodes) the gut is lacking • Osmoregulatory system is comprised of flame cells or protonephridia; water and waste exit the body through pores called nephridiopores General Characteristics cont. •Most species are monoecious, with cross fertilization • In some turbellarians and cestodes groups sperm transfer is by hypodermic impregnation Platyhelminths Systematics Traditional Classification Scheme 1. Class Turbellaria: Most free-living worms from terrestrial, FW and marine environments 2. Class Monogenea: All parasitic, mainly ectoparasites of fishes (e.g. found on the skin and gills) 3. Class Trematoda: All parasitic, mainly in the digestive tract of vertebrates a. Subclass Digenea - at least a 2 host life cycle; first host is a mollusc b. Subclass Aspidogastrea - most with only one host (mollusc) c. Didymozoidea - tissue dwelling parasites of fish 4. Class Cestoidea: All parasitic in all classes of vertebrates except agnathans; most have an intermediate host Characteristics of Revised Classification Scheme • The subphylum Catenulida is a sister group of the “true” Platyhelminthes • The main structural feature separating the subphylum Catenulida from the subphylum Euplatyhelminthes is the presence of a frontal organ in all the latter; Euplatyhelminthes also possess dense dermal ciliature compared to catenulids Revised Classification Scheme cont. • Among the S.P. Euplatyhelminthes, the acoelomate forms (Superclass Acoelomorpha) lack a protonephridia and generally do not have a gut • The Superclasses Rhabditophora have some kind of protonephtridia Taxonomic groups that will be emphasized Superclass Rhabditophora Class Trematoda Subclass Aspidobothrea Subclass Digenea Class Monogenea Class Cestoidea Subclass Cestodaria Subclass Eucestoda