A class of some 6000 described species of flatworms (see platyhelminthes) comprising the tapeworms—ribbon-like parasites within the gut of vertebrates. Tapeworms are surrounded by partially digested food in the host gut so they are able to absorb nutrients through their whole body surface. The body consists of a scolex (head), bearing suckers and hooks for attachment, and a series of proglottids (or proglottides), which contain male and female reproductive systems. The life cycle of a tapeworm requires two hosts, the primary host usually being a predator of the secondary host. Taenia solium has humans for its primary hosts and the pig as its secondary host. Mature proglottids, containing thousands of fertilized eggs, leave the primary host with its faeces and develop into embryos and then larvae that continue the life cycle in the gut and other tissues of a secondary host (see bladderworm).