flatworms

Bilateral symmetry comes with two key changes

Polarity development (ant vs post)
Centralization of sensory cells/ organs (CNS)

Flatworms

First bilateral animals
First triploblastic animals (mesoderm)

Most unique structure in flatworms

Rhabdites

Rhabdites

Rod like structures
"Mucus" producers
Help with locomotion/ prevention of desiccation

Flatworm feeding

Incomplete gastrovascular system (no anus)
Intestine with diverticula

Flatworm excretion

By diffusion and protonephridia

Primitive kidney cells in flatworm

Flame cells
(Filter and get rid of residuals)

Flatworm reproduction

asexual and sexual

Asexual

By fission with high capacity of regeneration

Sexual

Hermaphrodites with cross fertilization capacity

Flatworm nervous system

Ganglions system with lateral nerve cords

Concentration of organs

Light sensitive ocelli on head

Parasitism

Metabolism changes drastically as a result of life history

Adaptation to parasitism

High reproductive ability
High ability to attach to the host
Simplification/ reduction of some systems

High reproductive ability

High fertility, life cycle synchronized with host

High ability to attach to the host

Holdfasts (suckers, hooks), resistance

Simplification or reduction of some systems

No need for independent metabolism (host provides)

Parasite avoid killing their host but

They are selfish

Types of parasites

Single host parasites
Multiple host parasites

Single host parasites

One life phase in host + free

Multiple host parasites

Different stages at different hosts + free

2 hosts

Intermediate host and final host

Intermediate host

Host holds eggs or larval parasite

Final host

Host holds the adult parasite

4 groups of flatworms

Turbellaria
Monogenea
Trematoda
Cestoda

Monogenea

Ectoparasites on fish gills
1 host

Monogenea attachment organ

Opisthaptor (external hooks)

Digenea / Trematoda (flukes)

Endoparasites of vertebrates

1 hosts

How do trematoda attach to host

By suckers

Trematoda epidermis

Modified to a syncytial tegument (cells fused with no membranes in between)

Trematoda example

Liver fluke

Liver fluke cycle

Uncooked fish ingested
Shed in human feces
Go in snail
Go in fish

Cestoda (tapeworm)

Endoparasites of vertebrates

1 host

Tapeworm attached by

Scolex (hooks/ suckers)

When infected with tapeworm

Removal of proglottids does not terminate the parasitic infection
(Surgery to remove scolex)

Each proglottid is a

Subunit with its own reproductive machinery

Digestive system reduced to maximize egg production

Each proglottid becomes a Gravid proglottid full of eggs

Beef tapeworm

Cows eat contaminated grass (eggs)
Human eats in beef

Limitation in flatworms

Solid parenchima does not provide the best protection/ flexibility