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
How are the phylum Mesozoa described?
Tiny endoparasites of invertebrates
Structurally very simple and composed of very few cells
Outer layer of ciliated cells enclosing internal mass reproductive cells
Two distinct groups- origin uncertain
How are the phylum Gastrotricha described?
500 spp
Acoelomate
Meiofaunal, marine and freshwater
Body surface with cuticle
Flattened ventral surface with cilia for locomotion
Body surface with bristles, spines and scales
Many adhesive tubes with duo gland system
No circulatory or gas exchange systems
Gut with mouth and anus, sucking pharynx, most feed on bacteria
What separates the phylum Gastrotricha?
Many adhesive tubes
Gut with mouth and anus
Body surface with ciliated bands
How is the phylum Gnathostomulida described?
Two main characterising features:
- Only one cilium per epithelial cell
- Mouth has a pair of jaw and a comb-like ventral plate
How is the phylum Chaetognatha described?
70 species, all marine all but one genus are planktonic
Predators on zooplankton ecologically important due to high abundance and voracious feeding
Macrofaunal- range from 2mm to 120mm
Elongate transparent body with head, trunk and postanal tail regions
Trunk and tail with lateral fins to aid active swimming and reduce sinking
How are the phylum Chaetognatha structured?
Head flanked by large chitinous hooks for prey capture
Toxin (tetrodotoxin) injected by spines to immobilise prey
Ventral side of head with large chamber leading to mouth, eyes on dorsal surface and hood that can be drawn to cover head
How are the phylum Rotifera described?
Small meiofaunal size
Mostly freshwater and free living (a few marine spp)
Can be major constituent freshwater plankton
Often abundant- 1000 per litre in freshwater
How is the phylum Rotifera structured?
Body ends in bifurcate foot with adhesive glands
Anterior with ciliary organ (corona or wheel organ)- used in feeding and locomotion
How are the phylum Acanthocephala described?
Endoparasites of vertebrates (especially fish) as adults
Larvae parasitic in arthropods
No gut
Protrusible proboscis armed with hooks for attachment
Small- usually a few mm in length
