Phylum Platyhelminthes

Phylum
Platyhelminthes
Phylum
=
Platyhelminthes
flatworms
Platyhelminthes diversity
• 20,000 known species
• Found in every habitat;
terrestrial, marine, freshwater
• Mode of living - free living or parasitic (majority)
Characteristics of Platyhelminths
1. Bilateral symmetry (Simplest bilateria)
2. Triploblastic
(Simplest animals composed of
three fundamental cell layers)
Dugesia
3. No body cavity (acoelomate)
Mesoderm filled with parenchyma cells
Organ level of organization and these organs are
embedded in the mesoderm
4. Dorso-ventrally flat ;
5. Oval shape to worm like body;
- soft bodied animals
- measures 1 mm - 30 cm in length
6. Incomplete digestive tract; opens with a mouth
and ends blindly. (No anus)
Some animals do not have
a digestive tract.
7. Hermaphrodite (monocious)
Internal fertilization
between two animals
A free swimming ciliated larval stage/s in their
life cycle (some)
Müller's larva
Miracidium larva
8. Excretory organs
Function: Excretion and osmoregulation
Protonephridia
A net work of water collecting tubules connected
to flame cells
When cilia beat; they move water into the tubules
and out the body through pores called nephridiopores
9. Nervous system –
-cephalized with cerebral ganglia (brain)
Cephalization and bilateral symmetry evolved together
-net work of longitudinal and lateral nerves
10. Do not have circulatory or respiratory organs.
Gas exchange is through simple diffusion
Phylum Platyhelminthes
• Divided into 3 Classes
– Turbellaria
– Trematoda (Monogenea & Digenea)
– Cestoda
Class Turbellaria
Nearly 3000 species
- Mainly free-living
few are parasitic or commensal
Bdelloura on horseshoe crab
A commensal
Class Turbellaria
Most are marine,
some inhabit fresh water
and moist terrestrial habitats.
They are benthic.
(bottom feeders)
Marine species are colourful.
Terrestrial turbellarians are
dull brown colour
- Body cover is a multi ciliated epidermis
- Sub-epidermal rhabdites are present
- There is a basement membrane to separate epidermis
from muscle layer
Rhabdites and gland cells secrete adhesive material
(mucous)
Latin term for Class Turbellaria - "whirlpool“
is to visual impression of their
epidermal ciliary activity
Locomotion/Movement
– Swimming or gliding over the substrate
by action of cilia on mucous trail
– Swimming, crawling, folding or leech-like
looping movements
by peristaltic action of muscles
Feeding and Digestion
– Ventral mouth
– Moveable pharynx
– Extracellular digestion
• mostly carnivorous
•
They have a variety of sensory cells and glands.
•
Ocelli (eyespots)
-to detect light, dark, movement
•
Chemoreceptors throughout epidermis
Reproduction
Asexual reproduction
1. budding
Asexual reproduction
2. Fission and regeneration
Asexual reproduction
3. Break into pieces and form a cocoon
Sexual reproduction
• Internal fertilization between two animals
• Fertilized eggs are attached to
undersurfaces of stones and plants.
• Eggs hatch into miniature adult forms;
(direct development)
• Simple life cycle
Marine forms, (not all) produce a ciliated
larva called a Müller's larva .
Anterior apical organ
It is free swimming.
(brain)
Eyespots
As the larva matures, the
compact body enlarges;
the lobes shrink and gradually
disappear.
Ciliary lobes (6)
Turbellarians are classified among 12 Orders.
Based on
- type of pharynx
- branching pattern of the gut etc….
Order Acoela
Mouth opens into a mass of loosely packed cells.
Convoluta has symbiotic
algae living beneath their
epidermis.
A mass of green colour Convoluta on the sandy beach
Order Tricladida
“Planarians”
• Digestive tract - Three Diverticula
• Scavengers or active predators
Bdelloura
Planaria
Order Polycladida
eg. Leptoplana
• Found in tropical coral reefs.
-often mistaken for sea slugs
• Free living; carnivorous; scavengers
highly branched gut
Importance of Turbellarians:
-Colorful marine polycladids in the aquarium trade
-Dugesia is a common laboratory animal
But the vast majority of turbellarians are of little
economic importance to humans.
Humans can threaten turbellarian populations
through pollution of aquatic habitats.
Polycladid
Convoluta
Bdelloura
Planaria
Class Trematoda -Monogenea • Ectoparasitic on aquatic animals;
- on the gills and skin of fish
- bladder of frog
Requires Single host
• They have an opisthapter.
• It has suckers, hooks and anchors
to attach on to the host body.
Polystoma
Gyrodactylus
Simple life cycle:
Egg
Ciliated larva
Adult
Class Trematoda – (Sub class Digenea)
•Endoparasitic in vertebrates
• They
have two suckers
- Oral
sucker around mouth
- Ventral sucker
(acetabulum)
• Digestive tract consists of a
terminal mouth, muscular pharynx and
two intestinal diverticula.
- Their body cover
For Protection and Absorption
- is a live tegument
- has a glycoprotein layer
- No cilia
•Nervous system is reduced.
•Excretory system is a bladder with an excretory
pore.
• The entire body is occupied by the reproductive
system.
• Liver fluke - Fasciola hepatica
(The sheep liver fluke)
•They require two hosts to complete the
life cycle (vertebrate and a snail)
Eg. Fasciola hepatica
Sporocyst and redia
Class Cestoda Diversity
• Endoparasites
• Adults are found only in vertebrate intestine
4000 species
General morphology
Highly modified for parasitic life
• Long, flat, tape-like
“Tapeworms”
Some species can reach lengths of up to 15 m.
• No digestive tract. No mouth
- absorption through tegument
Body cover
–Tegument with glycoprotein layer
(similar to that of trematodes)
They have no sense organs
Adult body
- scolex (head)
- neck
- strobila - repeated segments
(proglottids)
Scolex consists hooks and suckers
New proglottids produce by neck region (Strobilization)
Each proglottid contains only reproductive organs.
Yolk gland
Mature proglottid
Mature reproductive organs
Fertilization
• - between two animals
• - between two proglottids of the same animal
• - within the same proglottid
Gravid proglottid
Eggs filled uterus
A gravid proglottid consists of uterus filled with
eggs. (male organs deteriorated)
There may be 80,000 eggs in one gravid proglottid.
Gravid proglottids detach from the body and pass
out with host faeces.
Taenia solium- Pork Tapeworm
Taeniorhynchus saginata - Beef tapeworm
Require two hosts – Definitive host - man
Intermediate host - pig/cow
Cysticercus
Onchosphere larva
Tapeworm