The Class Cestoda contains about  4,000 species of tapeworms, all of which are highly modified endoparasites that live in just about every vertebrate species.  Although too many of them can sap the strength of their hosts, tapeworms normally do little harm.  The long (up to 10 meters) flattened body (which is referred to as the strobila) is divided into segments called proglottids.  Most forms have an organ called a scolex at the anterior end with suckers, hooks, etc. that attach to the wall of the gut and prevent them from being swept away.  Tapeworms lack a digestive system and feed by absorbing nutrients directly from the host. The entire body surface is covered with minute projections called microtriches that greatly increase the absorptive surface area of the tapeworm. Tapeworms also secrete substances that inhibit the digestive enzymes of their host as well as lowering the pH around them to a level that they but not the digestive enzymes of their host can function. In tapeworms, much of the strobila is given over to reproduction. Each proglottid is monoecious, and cross fertilization or even self-fertilization is common. Proglottids can be filled with up to 100,000 eggs!