Cell Structure of the Flatworm
Flatworms are members of the invertebrate phylum Platyhelminthes and, true to their name, they are dorsoventrally flattened. There are 25,000 known species of flatworms. Most of these species are parasitic, have no specialized circulatory system, and absorb oxygen through diffusion. Flatworms are found inside other organisms and in many freshwater and saltwater environments.
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Types
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Flatworms are divided into four groups. The Turbellarians are free-living and found in streams, ponds, moist soils, and the sea. The Tremotada (flukes), Cestoda (tapeworms), and Monogenea, are parasitic and can be found inside other organisms for large parts of their lifecycles. According to scientists at Purdue University, the inability of flatworms to make their own fatty acids and sterols may explain their parasitism.
Body Form
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According to scientists at Bellarmine University, the flatworm was the first species to develop a mesoderm, the tissue between the ectoderm and endoderm in which organs and muscles grow. Unlike in annelid worms, the tapeworm body is not divided into segments, and there is no internal cavity or coelom.
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Digestive System
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The digestive tract of the flatworm has only one opening, the mouth, through which food is taken in, and waste is ejected. The most common flatworms are the three-branched flatworms or triclads. In the triclads, one branch of the gut travels to the head while the other two branch back into the body. The mouth is situated underneath, on the ventral surface.
Nervous System
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According to scientists at the University of Washington, the flatworm nervous system is made up of a nerve net connected by nerve cords, which are, in turn, connected to the cerebral ganglia in the head. This sort of nervous system is described as a ladder because of the formation of the nerves and nerve cord. Auricles--nerve structures on the side of the head--are equipped with chemoreceptors for finding food.
Reproductive System
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Most flatworms are hermaphroditic, containing both male and female reproductive structures, and are able to self-fertilize. Some species have structures that prevent self-fertilization whereas others, particularly the parasitic varieties, employ self-fertilization as the sole means of reproduction. According to scientists at Purdue University, some species of flatworm practice hypodermic fertilization, achieving sperm transfer by piercing the skin with the male cirrus near the female reproductive tract.
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References
Resources
- Photo Credit Flatworm image by Juergen Rudorf from Fotolia.com
