Well, this was definitely a challenging case this week! Many of you wrote in with suggestions that this is some type of fly larvae, and that is certainly a good thought. However, there are no structures such as spines or mouth parts to confirm this identity, and the history would be a bit odd.
Instead, this is actually a proglottid of Dipylidium caninum, the double-pored dog tapeworm. Although classically likened to "grains of rice," mature and gravid proglottids have been described as "pumpkin seed" shaped (as seen here).
The diagnosis is confirmed by identifying the characteristic egg packets of D. caninum. This can be accomplished by gently expressing eggs out of the proglottid using a sterile applicator stick.
Any surrounding fluid can then examined microscopically. In this case, the following eggs were seen.
Congratulations to Fred P. who got this one correct!