Protoscolices

A protoscolex is the larval stage of a tapeworm. It is essentially an immature tapeworm scolex (head) that is invaginated (turned inside out) within a fluid-filled cyst. This cyst, often called a hydatid cyst, is a metacestode stage that develops in an intermediate host. The protoscolex is the structure that will eventually develop into the adult tapeworm if the intermediate host is consumed by a definitive host.

Protoscolices are typically spherical or oval and possess a scolex at one end, complete with a rostellum (a hook-bearing projection) and a ring of suckers. These structures are fully formed but not yet functional in nutrient absorption or reproduction. The protoscolex can survive in this invaginated state for extended periods within the cyst. When the cyst is ingested by a suitable definitive host, such as a canid for *Echinococcus granulosus*, the protoscolex everts from the cyst. It then attaches to the intestinal wall and matures into an adult tapeworm, beginning the tapeworm's life cycle. The number of protoscolices within a hydatid cyst can vary greatly, from a few to thousands. Some species of tapeworms produce brood capsules within the hydatid cyst, which contain numerous protoscolices.