1. Chapter 13 - Introduction to Phylum Platyhelminthes

2. 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

3. 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

4. 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

5. General Characteristics cont.
Most species are monoecious, with cross fertilization
In some turbellarians and cestodes groups sperm transfer is by hypodermic impregnation

6. 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

7. 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

8. 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

9. Taxonomic groups that will be emphasized
Superclass Rhabditophora
Class Trematoda
Subclass Aspidobothrea
Subclass Digenea
Class Monogenea
Class Cestoidea
Subclass Cestodaria
Subclass Eucestoda