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Nemerteans and Turbellaria Phylum Plateyhelminthes Comparison of Reproductive Systems Abstract Nemerteans and the Plateyhelminthes class turbellaria are very similar in appearance But on closer investigation they are quite different in structure If the reproductive systems are examined it is found that they are different in many ways The first major difference is that nemerteans are mostly dioecious and turbellaria are monoecious Another difference is that external fertilization usually takes place in nemertean species and opposed to internal fertilization which takes place in turbellarians The last difference is that there is a free-living larval stage in some nemerteans and there is not a free-living larval stage in most turbellaria Along with the differences there are a couple similarities between the two groups The first is that they both have regenerative capabilities and the second is that they both can have a seminal vesicle for sperm storage and ejaculatory purposes Nemerteans are comprised of a small group of elongated unsegmented soft-bodied worms There are approximately 900 described species which mostly inhabit marine environments They are commonly found in shallow-water environments where they crawl over substrate burrow into the sediment or hide under stones rocks or mats of algae Pechenik 1996 In several aspects nemerteans superficially resemble free-living flatworms phylum Plateyhelminthes class Turbellaria But if the systems of the two groups are compared we see that there are differences between them For the purpose of this brief the reproductive systems of nemertea and the class turbellaria of the phylum plateyhelminthes will be compared Most of the nemertea species are oviparious and possess separate sexes although there are a few species that are hermaphroditic -Prosorhochmus claparedii combines hermaphroditism with ovoviviparity and anal parturition The young worms emerge from the gonads to enter the alimentary tract Gibson 1982 The gonads are found in the intestinal region where they form two lateral rows which are serially repeated alternating singly or in groups with the
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