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