Phylum Platyhelminthes

Name? From 2 Greek words:‘platy’ meaning flat and ‘helminth’ meaning worm

Characteristics:

• Have tissues and internal organs• Have 3 embryonic germ layers – endoderm, mesodermand ectoderm• Bilateral symmetry• Cephalization• Acoelomates – No body cavity persent

3 Main Groups or Classes:

1. Turbellaria – Free living, most live in marine orFresh water.Example: Planaria (Fresh water)

Marine dwelling:

Some live on the forest floor:

2. Class Trematoda (Flukes):Parasitic flatworms often living in the host’sinternal organs. Some are ectoparasites on the host’s skin, mouth, gills, etc.

Liver fluke in deer

3. Class: CestodaTapeworms – Long, flat parasitic wormsadapted to life inside the intestines of their hosts.

Characteristics:• Have tissues and internal organs• Have 3 embryonic germ layers – endoderm, mesodermand ectoderm• Bilateral symmetry• Cephalization• Acoelomates – No body cavity persent

Specialized structures:

• Free living flatworms: Ganglia (groups of nerve cells)that control a nervous system – primitive ‘brain’.

• Also have eyespots to detect light with

Free living flatworms have mouth with a pharynxthat pumps food into the digestive cavity.

• Free living flatworms have muscle cells to aid in movement

• Some flatworms have flame cells that help them removeexcess water and also wastes like ammonia and urea from the body. (Help with excretion)

Parasitic worms such as tapeworm:• Head (scolex) contains several hooks or suckers

that are used to attach to the intestinal wall of the host

Mode of nutrition:

Free living flatworms: Predators or scavengers – eatmolluscs, protozoans, crustaceans, some live withalgae that provide them with carbohydrates.Use pharynx to pump in food into the mouth. Have a digestive cavity or gut.

Parasitic flatworms: Feed on blood, tissue or piecesof cells within the host’s body. Tapeworms haveno digestive cavity – simply absorb nutrients that Are already in the host’s intestines.

Movement:

Free-living: Use cilia and muscle cells to glide, twistand turn.

Reproduction:

Free living: Hermaphrodites and reproduce sexually.Sperm are delivered between 2 flatworms. Eggs arelaid and hatch in a few weeks.• Also reproduce asexually through fission.

Parasitic flatworms:

They have complicated life cycles that can involveboth sexual and asexual reproduction.

http://science.jburroughs.org/mbahe/BioA/starranimations/chapter23/videos_animations/tapeworm.html