Answer: Hydatid sand of Echinococcus sp.; mostly likely Echinococcus granulosus due to the abundant protoscoleces/hooklets and presence within a single cyst.
The photo from this case shows numerous free hooklets in the background of degenerating protoscoloces. As Blaine pointed out, the mixture of disintegrating protoscoleces with free hooklets and calcareous corpuscles is often referred to as "hydatid sand."
Echinococcus species are one of several members of the Taeniidae that can cause human disease. Echinococcus granulosus is now recognized as a complex of closely-related organisms, with E. granulosus sensu stricto being the most common species causing human disease worldwide. Other members of this complex that cause human disease are E. canadensis and E. ortleppi. Differentiation of the members of the E. granulosus complex is primarily accomplished through molecular means.
E. multilocularis human infection is fortunately less common, as this parasite is known for its more aggressive and invasive growth pattern. As several readers pointed out, the cysts of E. multilocularis invade host tissue, much like a tumor, and are not contained within a large parent cyst. Protoscoleces are not commonly seen in human infections with E. multilocularis. Finally, E. oligarthra and E. vogeli are rare causes of human echinococcosis in South and Central America.