Download Platyhelminthes – gap notes

Phylum Platyhelminthes
Embryonic Development and Body Cavities
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Important ways that we classify animals into different groups is based on features of their embryonic
development, and the formation of body cavities (called a coelom).
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Pages 187-190 of your study guide (32.1-32.2) contains helpful diagrams on this topic.
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The division of animals into protostomes and deuterostomes is based on events in early development.
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Protostomes are animals in the phyla Platyhelminthes, Nematoda, Annelida, Arthropoda, and Mollusca.
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Deuterstomes include animals in the phyla Echinodermata and Chordata.
The Blastula
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A zygote will undergo mitotic divisions and form what is called the blastula
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The blastula a hollow ball made from cells.
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One end of the blastula then folds inwards in a process known as gastrulation.
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The hole open to the outside is called the blastopore.
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The cell layer forming the inside pouch is called the endoderm.
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The cell layer on the outside is the ectoderm.
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A third layer of cells forms what is called the mesoderm.
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These cell layers will eventually develop into specific kinds of tissues (ectoderm will form skin and
nervous tissue, endoderm will form the digestive tract, the mesoderm will form muscles and blood cells)
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If the blastopore becomes the mouth, the animal is a protostome.
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If the blastopore becomes the anus, the animal is a deuterostome.
Body Cavities
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The germ layers (endoderm, ectoderm and mesoderm) also play a role in the kinds of body cavities
formed in animals.
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The body cavity is a fluid filled space separating the digestive tract from the outer body wall.
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A body cavity (AKA coelom) provides cushioning to internal organs, enables the internal organs to
grow and move independently of the outer body wall, and can act like a skeleton.
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Simple animals are called acoelomates, which means they lack a body cavity.
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Pseudocoelomates are animals with a body cavity that is partially lined with a layer of cells
originating from the mesoderm.
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Animals with a true body cavity are called coelomates.
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A true body cavity means that it is completely lined tissues which originated from the mesoderm
layer.
Platyhelminthes: The Flatworms
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Flatworms are the simplest animals with bilateral
symmetry and cephalization (they have a head).
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Flatworms have more developed organ systems than
sponges and Cnidarians.
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Platy means flat and helminth means worm
Feeding
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Some flatworms hunt or scavenge for food.
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These kinds of flatworms have a muscular tube called a pharynx that they use to suck food into the
gastrovascular cavity.
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Digested food can diffuse throughout the body because the intestine branches throughout the entire
body.
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Flatworms will expel undigested food back through their mouths
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Some flatworms hunt or scavenge for food.
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These kinds of flatworms have a muscular tube called a pharynx that they use to suck food into the
gastrovascular cavity.
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Digested food can diffuse throughout the body because the intestine branches throughout the entire
body.
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Flatworms will expel undigested food back through their mouths
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Other types of flatworms obtain nutrients by being parasites (such as tapeworms).
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Most parasitic flatworms have simpler organ systems than free-living flatworms that hunt or
scavenge.
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Tapeworms that live in a host’s intestine do not have a digestive tract at all because they can absorb
the food as it passes by.
Respiration and excretion
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Flatworms don’t have a specialized circulatory or respiratory system.
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They are small enough that they can get the oxygen and nutrients into their tissues by diffusion, as
well as expel carbon dioxide and metabolic wastes the same way.
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Some flatworms have special organs to help with excretion of wastes.
Nervous System and Sensory Organs
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Free-living flatworms have a simple brain, or ganglia.
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Long nerve cords travel from the brain down the length of the body and shorter nerve cords travel
across the body.
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Ocelli, or eyespots can detect light and darkness.
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Special sensory cells also allow flatworms to detect chemicals in their environment, such as chemicals
found in food.
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The nervous system of parasitic flatworms is often simpler (or non-existant) than their free living
relatives.
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Why might this be? _______________________________________________________
Reproduction
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Free-living flatworms can reproduce sexually and asexually.
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Flatworms are hermaphroditic, which means a single flatworm has both male and female sex
organs.
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Asexual reproduction occurs by fission in free-living flatworms.
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Some flatworms can break into several pieces with each growing into a new worm.
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Parasitic flatworms typically only reproduce sexually.
The 4 Classes of Platyhelminths
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Turbellaria – Free-living
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Trematoda – Parasitic
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Monogenea - Parasitic
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Cestoda - Parasitic