While it is far from a universal rule that the largest worms are found in the largest hosts, it is true that the longest tapeworm known is from a whale. And this may be the candidate for the smallest tapeworm- Protogynella sp. from a shrew, Sorex vagrans. It is only 250 microns (a quarter of a millimeter) long. Shrews are among the smallest mammals (some weigh less than 5 grams) and most of their parasites (trematodes, cestodes, nematodes) are correspondingly tiny. The darker staining structures are the eggs, which probably infect an arthropod intermediate host since shrews are primarily insectivorous.
Contributed by Mike Kinsella.