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My text book says:

Platyhelminthes are unsegmented worms.

Moreover, tape worm (Taenia solium), which is segmented, is one of the examples of Platyhelminthes.

My question is: why a segmented worm is included as an example in Platyhelminthes (unsegmented worm)?

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    $\begingroup$ Welcome to Biology.SE. Could you please add the reference to your textbook? Thanks! $\endgroup$ – AlexDeLarge Jun 2 '17 at 14:29
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The confusion here is quite common (at least it was quite common among my students): the "segments" in a tapeworm like Teania, properly called proglottids, are not really segments, or metamers.

According to Hickman et al. (2001), in the chapter 17, Segmented Worms:

Body segmentation, or metamerism, in annelids is not merely an external feature but is also seen internally in the repetitive arrangement of organs and systems and in the delimiting of segments (also called metameres or somites) by septa. Metamerism is not limited to annelids; it is shared by arthropods (insects, crustaceans, and others), whose metamerism may be homologous to that in annelids, and also by vertebrates, in which it evolved independently.

Some pages before, in chapter 14 (Acoelomate Bilateral Animals), the authors say:

Some zoologists have maintained that the proglottid formation of cestodes represents “true” segmentation (metamerism), but we do not support this view. Segmentation of tapeworms is best considered a replication of sex organs to increase reproductive capacity and is not related to the metamerism found in Annelida, Arthropoda, and Chordata (see pp. 193 and 371).

I'm citing Hickman, but all zoology books I'm aware of (Ruppert-Fox-Barnes, Brusca & Brusca, etc...) will say the same: segmentation is synonym of metamerism, and Plathyhelminthes (including Teania) are not segmented animals.

Source:

  • Hickman, C., Roberts, L., Larson, A., Ober, W. and Garrison, C. (2001). Integrated principles of zoology. 1st ed. Boston: McGraw-Hill.
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  • $\begingroup$ ...sir,also tell me that at one page in my text book it has been written that Nervous system is well developed in platyhelminthes but at other;there mentioned a line (under heading of adaptation of flat worms );that there is degeneration of nervous system and muscles....What is truth?both line are contradict to each other...please make it clear! $\endgroup$ – M.Naeem Ahmad Jun 2 '17 at 14:46

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