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.
- Hickman, C., Roberts, L., Larson, A., Ober, W. and Garrison, C. (2001). Integrated principles of zoology. 1st ed. Boston: McGraw-Hill.