Answer: Protoscoleces and hooklets of Echinococcus spp.
The fact that this is a single cystic mass, and that protoscoleces are present suggests that this is E. granulosus instead of E. multilocularis, but clinical and radiographic correlation is necessary for definitive speciation. E. granulosus typically produces a single, slowly growing cyst, that may eventually contain one or more daughter cysts - each containing protoscoleces (the so called 'cysts within a cyst' appearance) - while E. multilocularis typically produces multiple, rapidly growing cysts that expand in a invasive fashion, and are not constrained by an outer cyst wall. E. multilocularis cysts rarely contain protoscoleces in humans, but are often 'sterile'. These 2 species are the most common to infect humans, but other species rarely infect humans as well.
Thank you all for the great comments on this case! I especially enjoyed "I am hooked on Echinococcus" by Scott and Anonymous.