Phylum Platyhelminthes

Platyhelminthes diversity

• 20,000 known species

• Found in every habitat;

terrestrial, marine, freshwater

• Mode of living - free living or parasitic (majority)

Phylum Platyhelminthes

= flatworms

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

A net work of water collecting tubules connected

to flame cells

Protonephridia

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

• Divided into 3 Classes

– Turbellaria

– Trematoda (Monogenea & Digenea)

– Cestoda

Phylum Platyhelminthes

Class Turbellaria

Nearly 3000 species

- Mainly free-living few are parasitic or commensal

Bdelloura on horseshoe crab

A commensal

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

Class Turbellaria

Rhabdites and gland cells secrete adhesive material

(mucous)

- Body cover is a multi ciliated epidermis

- Sub-epidermal rhabdites are present

- There is a basement membrane to separate epidermis

from muscle layer

-

Latin term for Class Turbellaria - "whirlpool“

is to visual impression of their

epidermal ciliary activity

– 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

Locomotion/Movement

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

• 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

Sexual reproduction

Marine forms, (not all) produce a ciliated

larva called a Müller's larva .

It is free swimming.

Ciliary lobes (6)

As the larva matures, the

compact body enlarges;

the lobes shrink and gradually

disappear.

Anterior apical organ

(brain)

Eyespots

Turbellarians are classified among 12 Orders.

Based on

- type of pharynx

- branching pattern of the gut etc….

Order Acoela

Convoluta has symbiotic

algae living beneath their

epidermis.

A mass of green colour Convoluta on the sandy beach

Mouth opens into a mass of loosely packed cells.

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.

Convoluta BdellouraPolycladid 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

- is a live tegument

- has a glycoprotein layer

- No cilia

For Protection and Absorption

•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 -

• Endoparasites

• Adults are found only in vertebrate intestine

4000 species

Diversity

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)

New proglottids produce by neck region (Strobilization)

Each proglottid contains only reproductive organs.

Scolex consists hooks and suckers

Yolk glandMature proglottid

Mature reproductive organs

Fertilization

• - between two animals

• - between two proglottids of the same animal

• - within the same 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.

Gravid proglottid

Taenia solium- Pork Tapeworm

Taeniorhynchus saginata - Beef tapeworm

Cysticercus

Onchosphere larva

Tapeworm

Require two hosts – Definitive host - man

Intermediate host - pig/cow