Answer: Echinococcus spp. hooklets. It is common for protoscoleces (the larval form of echinococcus) to degenerate, releasing free hooks and cellular debris into the cyst fluid. This material comprises the 'hydatid sand' or gritty material in aspirated cyst contents. Note the characteristic shape and refractile nature of the hooklets.
The differential diagnosis includes hooklets from a cysticerca (larval form of Taenia solium) which are very similar in appearance to the hooklets of Echinococcus spp. However, a cysticerca contains only one larva which does not commonly degenerate and release its hooklets. Also, the size of the cysticerca cyst is much smaller than that of echinococcus and contains very little fluid, making aspiration impractical.