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

The Superclasses Rhabditophora have some kind of protonephtridia 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