Flatworms and relatives

Question 1

How are the phylum Xenacoelomorpha described?

Answer 1

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

Question 2

What are the characteristics of the platyhelminths?

Answer 2

• 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)

Question 3

What classes exist in the phylum platyhelminths?

Answer 3

• Turbellarians- mostly free-living flatworms-4500 spp
• Trematodes (flukes)
• Cestodes (tapeworms)

Trematodes and Cestodes entirely parasitic and derived from one turbellarian group

Question 4

What are the characteristics of the class Turbellarians?

Answer 4

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

Question 5

What is an example of a turbellarian?

Answer 5

Candy striped flatworm- Prostheceraeus vittatus (polycladia)

Question 6

At the cellular level what are some characteristics of the Turbellarians?

Answer 6

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)

Question 7

What system is seen in most groups of Turbellarians?

Answer 7

Duo-gland adhesive system- most groups

Question 8

What are Rhabdites?

Answer 8

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

Question 9

Are spicules present in Turbellarians?

Answer 9

spicules are present in some species
Supportive function

Question 10

How is the gut structured in the Turbellarians?

Answer 10

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

Question 11

Why is the gut divided in the Turbellarians?

Answer 11

Gut divided to:
- Increase surface area for absorption
- Compensate for lack of internal transport system

Question 12

What are some examples of gut complexity in orders of the Turbellarians?

Answer 12

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

Question 13

Where is the mouth located on the Turbellarians?

Answer 13

Mouth usually on mid ventral line

Question 14

What are the mouth parts found in the Turbellarians?

Answer 14

Simple pharynx- simple ciliated tube (smaller turbellarians)

Pilicate pharynx- folded pharynx or muscular pharyngeal tube within pharyngeal cavity

Bulbous pharynx- eversible muscular bulb

Question 15

What is an example of the complexity of the mouth in the Turbellarians?

Answer 15

Sub-order- Kalyptorhynchia- have anterior proboscis with ‘jaws’- not part of mouth or gut

Question 16

How is the nervous system structured in the Turbellarians?

Answer 16

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

Question 17

How are the eyes structured in the turbellarians?

Answer 17

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

Question 18

How do the Turbellarians reproduce?

Answer 18

Asexual:
- By fission of various types
- Consequent remarkable regenerative powers

Sexual reproduction:
- Hermaphrodites
- Sperm transfer by copulation

Question 19

How are the class Trematoda (flukes) described?

Answer 19

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

Question 20

What is the most significant orders in the class Trematoda?

Answer 20

Order- Monogenea
Order- Digenea

Question 21

How is the order Monogenea described?

Answer 21

Mostly ectoparasites of fish
Single host in life cycle
Large posterior adhesive organ (opisthaptor)

Question 22

How is the order Digenea described?

Answer 22

> 6,000 spp
Mostly endoparasites of vertebrates
Multiple hosts in life cycle
Some are a serious problem to livestock and humans

Question 23

How are the class Cestoda (tapeworms) described?

Answer 23

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

Question 24

How are the class Cestoda structured?

Answer 24

Anterior scolex with attachment organs, neck region, chain of segments (proglottids) budded off neck region
Reproductive system repeated in each proglottid

Question 25

How are the phylum Mesozoa described?

Answer 25

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

Question 26

How are the phylum Gastrotricha described?

Answer 26

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

Question 27

What separates the phylum Gastrotricha?

Answer 27

Many adhesive tubes
Gut with mouth and anus
Body surface with ciliated bands

Question 28

How is the phylum Gnathostomulida described?

Answer 28

Two main characterising features:
- Only one cilium per epithelial cell
- Mouth has a pair of jaw and a comb-like ventral plate

Question 29

How is the phylum Chaetognatha described?

Answer 29

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

Question 30

How are the phylum Chaetognatha structured?

Answer 30

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

Question 31

How are the phylum Rotifera described?

Answer 31

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

Question 32

How is the phylum Rotifera structured?

Answer 32

Body ends in bifurcate foot with adhesive glands
Anterior with ciliary organ (corona or wheel organ)- used in feeding and locomotion

Question 33

How are the phylum Acanthocephala described?

Answer 33

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