Answer: Enterobius vermicularis (pinworm)
Shown are cross-sections of the adult worms within the lumen of the appendix. In some cross-sections you can see the characteristic eggs within the females (below is the egg in a stool iodine prep from a different patient).
What is also classic about this case is the location (appendix and cecum are traditional locations for the adult female pinworm) and the lateral alae on the worms. Alae, (plural of ala, Latin for 'wing') are longitudinal ridges seen in the larval forms of nematodes, (e.g. Ascaris lumbricoides), and the adult worm Enterobius vermicularis. In fact, Enterobius is the only ADULT worm with these ridges that parasitizes humans. Below is the cross-section from this case showing the lateral alae (arrows) as well as a longitudinal view of the adult worm in an iodine stool prep (arrow marks the ridge on one side of the worm). The ridges extend the length of the worm.
Tom was correct in mentioning Toxocara spp. since the larval form causes visceral larva migrans in humans. However, the larva are much smaller than the adult E. vermicularis and would be seen within the parenchyma of tissues (e.g. liver) in association with a marked inflammatory response.