Molecular testing showed this to be Taenia saginata.
There was a lot of great discussion regarding the differential on this case. Some of the key features were as follows:
1. Large size - although not given, you can see that this is a large tapeworm that fills the Petri dish. Even if this was a small Petri dish, the size would be most consistent with Taenia spp., Diphyllobothrium spp., and possibly Hymenolepis diminuta.
2. Size/shape of proglottids - this is one area where the ID gets tricky. Mature proglottids of H. diminuta and Diphyllobothrium are wider than they are long, whereas mature proglottids of Taenia spp. are longer than they are wide. The caveat is that immature proglottids of Taenia (as seen in this case) are also wider than they are long. Thus all 3 of these worms are in the differential. However, the proglottids of H. diminuta are very short and not like the ones shown in this case. Therefore, this diagnosis is less likely, leaving us with Diphyllobothrium and Taenia species as considerations.
3. Internal proglottid structures - this is a very helpful feature of this case since an internal 'rosette-shaped' uterus (consistent with Diphyllobothrium) is NOT present, but instead the opening to the lateral genital pore can be seen; the latter is consistent with this being a Taenia sp. proglottid.
Taenia sp. (this case); arrows show openings to lateral genital pores:
H. diminuta showing extremely short proglottids (from CDC DPDx):
4. Morphology of the eggs - another tricky factor here! This egg clearly contained a hooked-onchosphere, but it lacked the outer shell of Hymenolepis and the outer striated wall of Taenia spp. As mentioned above, this was a Taenia sp., and thus the lack of the thick striated outer shell can be attributed to the immature state of the egg (matching the immature proglottids).