Transcript Chapter 13 - Introduction to Phylum Platyhelminthes
Chapter 13 - Introduction to Phylum Platyhelminthes
General Characteristics
• Bilaterally symmetrical; dorsoventrally flattened • Acoelomates; most of the body is comprised of running through the parenchyma
parenchyma,
a collection of fibers and cell types (sensory, food storage, etc.); internal organs are embedded in the parenchyma; muscle fibers are also found • 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 nephridiopores
; water and waste exit the body through pores called
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 3. Class Trematoda
: All parasitic, mainly ectoparasites of fishes (e.g. found on the skin and gills) : 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
(mollusc)
c. Didymozoidea
- most with only one host - 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 from
subphylum Catenulida
catenulids is a sister group of the “true” Platyhelminthes • The main structural feature separating the subphylum Catenulida
the subphylum Euplatyhelminthes
is the presence of a frontal organ in all the latter; Euplatyhelminthes also possess dense dermal ciliature compared to
Revised Classification Scheme cont.
• Among the S.P. Euplatyhelminthes, the acoelomate forms (
Superclass Acoelomorpha
gut • The
Rhabditophora
) lack a protonephridia and generally do not have a
Superclasses
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