Flatworms and relatives Flashcards
How are the phylum Xenacoelomorpha described?
Acoel flatworms
Small structurally simple flatworms. Ciliated epidermis, acoelomate. Most lack a gut- intestinal cells form solid syncitial mass
Formerly regarded as a sub-group of the Platyhelminth phylum
Some have symbiotic algae
What are the characteristics of the platyhelminths?
- Bilaterally symmetrical
- Acoelomate
- Digestive tract with single opening or absent
- Dorso-ventrally flattened
- Most with hermaphroditic reproductive system
- Freeliving representatives (turbellarians) and many parasitic forms (tapeworms and flukes)
What classes exist in the phylum platyhelminths?
- Turbellarians- mostly free-living flatworms-4500 spp
- Trematodes (flukes)
- Cestodes (tapeworms)
Trematodes and Cestodes entirely parasitic and derived from one turbellarian group
What are the characteristics of the class Turbellarians?
Body shape oval to elongate
Flattened- more pronounced in larger worms
Some with head projections- tentacles or auricles
Most are <1cm (range from microscopic to >60cm)
Most are marine and of these, most are benthic and cryptic- a few are pelagic
Some freshwater species
Some are terrestrial but confined to humid environments
Mostly predators or scavengers
What is an example of a turbellarian?
Candy striped flatworm- Prostheceraeus vittatus (polycladia)
At the cellular level what are some characteristics of the Turbellarians?
Ciliated epidermis (in some triclads and polyclads the cilia are restricted to the ventral surface)
Gland cells- characteristic feature of the turbellarians (function in adhesion to substrate and in prey capture)
What system is seen in most groups of Turbellarians?
Duo-gland adhesive system- most groups
What are Rhabdites?
Typical of almost all species of turbellarians
Rod-like epidermal bodies produced by epidermal gland cells
Function uncertain- discharged for defence +/or generate slime for movement
Are spicules present in Turbellarians?
spicules are present in some species
Supportive function
How is the gut structured in the Turbellarians?
Gut- varying complexity- may be simple (small species)
Mouth serves for ingestion and egestion
Wall of gut single layer of phagocytic and gland cells
Some groups with ciliated intestine
Why is the gut divided in the Turbellarians?
Gut divided to:
- Increase surface area for absorption
- Compensate for lack of internal transport system
What are some examples of gut complexity in orders of the Turbellarians?
Order Polycaldida- Gut with multiple branches off single main tube
Sub-order Tricaldida- gut with 3 main branches (1 anterior, 2 posterior lateral) and multiple side branches
Where is the mouth located on the Turbellarians?
Mouth usually on mid ventral line
What are the mouth parts found in the Turbellarians?
Simple pharynx- simple ciliated tube (smaller turbellarians)
Pilicate pharynx- folded pharynx or muscular pharyngeal tube within pharyngeal cavity
Bulbous pharynx- eversible muscular bulb
What is an example of the complexity of the mouth in the Turbellarians?
Sub-order- Kalyptorhynchia- have anterior proboscis with ‘jaws’- not part of mouth or gut
How is the nervous system structured in the Turbellarians?
Nervous system:
- Simple epidermal nerve net (in some acoels)
- 2 or 3 longitudinal cords, nerve net and brain
- More advanced groups- increased prominence of ventral pair of cords
How are the eyes structured in the turbellarians?
Eyes:
- Common- usually 2 but sometimes numerous
- Most of pigment cup type
- A few acoels just have patches of photo receptors and pigment cells in epidermis
How do the Turbellarians reproduce?
Asexual:
- By fission of various types
- Consequent remarkable regenerative powers
Sexual reproduction:
- Hermaphrodites
- Sperm transfer by copulation
How are the class Trematoda (flukes) described?
Entirely parasitic
Appearance similar to turbellarians
Outer layer of body is a nonciliated cytoplasmic syncytium
Ventral and oral adhesive organs are characteristic
Mouth is typically anterior
Gut with single opening (mouth) and one or two branches
What is the most significant orders in the class Trematoda?
Order- Monogenea
Order- Digenea
How is the order Monogenea described?
Mostly ectoparasites of fish
Single host in life cycle
Large posterior adhesive organ (opisthaptor)
How is the order Digenea described?
> 6,000 spp
Mostly endoparasites of vertebrates
Multiple hosts in life cycle
Some are a serious problem to livestock and humans
How are the class Cestoda (tapeworms) described?
Entirely parasitic- gut parasites of vertebrates
Life cycle involves an intermediate host
Appearance dissimilar to turbellarians and trematodes
Outer layer of body is a nonciliated cytoplasmic syncytium
Gut absent
How are the class Cestoda structured?
Anterior scolex with attachment organs, neck region, chain of segments (proglottids) budded off neck region
Reproductive system repeated in each proglottid
