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Kingdom Animalia, Phylum Platyhelminthes (Flatworms)

Three embryonic germ layers
Bilateral symmetry
Cephalization (head)

Coelom (Greek for cavity or hollow) = a fluid filled body cavity
Acoelomates = without coelom

Form and Function in Flatworms


Free-living = carnivores or scavengers
Have digestive cavity, mouth, pharynx

Parasites: = Feed on blood, tissue fluids, or pieces of cells from within a host

Most do not have a complete digestive system because they absorb digested material directly

Respiration, Circulation, and Excretion:

Thin bodies allow for materials to diffuse (resp, excrection..etc)

Flame cell = specialized cells that remove excess water


Ganglia = group of nerve cells control the nervous system (like a brain)

Eyespot = group of cells that can detect light


Flatworms moves in 2 ways
1) Cilia = helps them glide through water and on stream floors
2) Muscle cells = twist and turn


Sexual Reproduction = Hermaphrodites = has both male and female reproductive organs
Asexual Reproduction by fission = organism spits in two


planarian1. Class Turbellarians = free-living flatworms
Fresh or marine water
Example: Planarians (cross-eyed)

Planarian(also known as Dugesia)--lives in freshwater
--mostly a scavenger, also feeds on protists
--they can regenerate (regrow parts), Reproduction by FISSION

Anatomy of the Planarian

Brain (ganglia) - planarian can process information about their environement
Pharynx - used for suckling food in (the mouth is at the end of the pharynx)
Eyespot - simple eye, can detect light
Flame cells - located along the lateral edges, used for excretion
Intestine - digestion (does not have an anus)


Investigation: Planaria and Regeneration

2. Class Trematoda = parasitic flatworms
a.k.a "flukes" live in mouth, skin, or gills of host

Primary host = the host in which a parasite reproduces sexually
Intermediate host = the host in which asexual reproduction occurs

Schistosoma mansoni - multiple host:
Primary host = human
Intermediate host = snail

Causes Schistosomiasis -in humans; decays lungs liver, spleen, or intestines. Tropical areas with poor sanitation/sewage.



3. Class Cestoda =tapeworms
Long, flat, parasitic
Live in intestines


Scolex = a structure that contains suckers and/or hooks
Proglottids = body segments of the tapeworm

Each mature proglottid is a hermaphrodite
Testes produce sperm, fertilize the eggs to produce a zygote
Zygotes are passed out through the feces.

A dormant, protective cyst is formed in the intermediate cyst.

This is why you should never eat incompletely cooked meat.