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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