Tapeworms (class Cestoda) are a type of worm and invertebrate that live as internal parasites of vertebrates, almost always in the intestinal tract. The life cycle usually involves one or two intermediate hosts, which may be invertebrate or vertebrate, depending on the species. The most familiar of these parasites is the beef tapeworm, whose intermediate host is a cow and whose final host is a human being. Tapeworms exhibit many special adaptations for their parasitic way of life. Like flukeworms, they have a resistant cuticle instead of the epidermis of their free-living ancestors. And they have secondarily lost both mouth and digestive tract. Bathed by the food in their host's intestine, they absorb predigested nutrients across their general body surface. Diffusion, probably augmented by active transport, suffices to provide the worm with all of the food it needs.
The head of a tapeworm is a small knoblike structure called a scolex, which usually looks like a doorknob. On the head of a tapeworm, there are normally suckers and/or hooks that allow the worm to attach itself to a host. Immediately behind the scolex is a neck region, which is followed by a very long ribbon-like structure that resembles a tail. Tapeworms can be found in length from 10 cm to 23m. This long body is usually divided by transparent constrictions in to a series of segments called proglottids. Each proglottid is essentially a reproductive sac, each containing both male and female organs for reproduction.