Answer: Pinworm (Enterobius vermicularis) infection. As Anonymous mentions, there is both a male and female in the top image. The male is much smaller and has a curved tail.
Besides the characteristic location (appendix and cecum), there are several features that allow for identification of these worms. First, the largest worms measure 1 cm or more in length, compared to the smaller 5 mm-long males (although I didn't give you this information in this case). Second, the females have a distinctive sharply pointed tail (pin-like) that comprises approximately 1/4 of their length. In both sexes, there are lateral alae (cephalic inflations) which are most marked on cross-section (and aren't easily seen in this case). Finally, they have a distinctive well-defined esophagus which is divided into a muscular and bulbous portions. The esophagus is nicely seen in this case, as illustrated below.