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Platyhelminthes (Flatworms)
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Flatworms are in the phylum Platyhelminthes. Most flatworms are dorsal ventrally flattened and have bilateral symmetry. Bilateral symmetry means an animal has a right and left side and is important in the evolution of animals because it allows for different parts of the body to have specialized structures, like a head with specialized sense organs, limbs, and other organ systems located in certain parts of the body.
Flatworms have three germ layers. Like cnidarians flatworms have a layer of ectoderm and endoderm, but also a third germ layer called mesoderm. In lab, we will be examining a live free living flatworm called Planaria, make sure you look for cephalization, which means they have a head with specialized sense organs (eyespots, auricles) to move head first through the environment. Further, flatworms have several organ systems, including a well developed excretory system, nervous system and reproductive system. Flatworms lack a respiratory and ciruclator system. Flatworms are acoelomate because they lack a body cavity (fluid filled layer) and their body construction is solid. |
Germ layer organ development:
Ectoderm:
Mesoderm:
Endoderm:
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Parasitic Tapeworm (Taenia) from lab
Free living Planaria from lab
Planaria eating egg yolk from labIn lab, we fed planaria egg yolk. Check out the pictures of the gastrovascular cavity on the right. One picture will be of a live planaria that has been starved (gastrovascular cavity is empty) and then the other pictures will have the yellow egg yolk granules in the gastrovascular cavity |
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Nematodes (Roundworms)
Roundworms are in phylum Nematoda and live in most aquatic habitats and damp soil. They are free living or parasitic and are major agricultural pests and can cause serious illness in humans. For example, the roundworm Anisakiasis is present in uncooked or raw fish (sushi) and can cause abdominal pain, distention, diarrhea, and mild fever. Other disease causing nematodes include pinworms and hookworms, which cause trichinosis and elephantiasis.
Nematodes have three germ layers, a body cavity between the mesoderm and endoderm (pseudocoelom), a well developed nervous system, are not segmented, and must molt to grow. Furthermore, the digestive tract is one way, food enters through the mouth and is excreted at the anus. The body of the nematode is covered with a thick, flexible cuticle, than must be molted (shed) when the animal grows. This thick, flexible cuticle, allows a nematode to move through the soil and also withstand the harsh environment of a digestive system.
Nematodes have three germ layers, a body cavity between the mesoderm and endoderm (pseudocoelom), a well developed nervous system, are not segmented, and must molt to grow. Furthermore, the digestive tract is one way, food enters through the mouth and is excreted at the anus. The body of the nematode is covered with a thick, flexible cuticle, than must be molted (shed) when the animal grows. This thick, flexible cuticle, allows a nematode to move through the soil and also withstand the harsh environment of a digestive system.
