Answer: proglottids of the broad fish tapeworm Diphyllobothrium latum.
Congratulations to everyone who got this right! Also in the differential would be proglottids of Taenia spp. and artifacts such as onion skin which may be passed relatively intact and mimic tapeworm segments. However, the diagnosis is easily made by examining the central proglottid structures which form a rosette-structure, or so-called "Chinese character" as shown here:
When stained, this structure is more apparent (as seen from a previous case of the week):