The cestodes, or tapeworms, differ in a number of ways from other flatworms. Their bodies are long and flat, made up of many segments called proglottids. Each proglottid is a reproductive unit, essentially a factory to produce gametes. Adults lack cilia and their surface is a tegument (as in monogeneans and trematodes), but in cestodes the tegument is covered with tiny projections, microvilli, which increase its surface area and thereby its ability to absorb nutrients from a host. Digestive tracts are absent completely. At the tapeworm's anterior end is a specialized segment called a scolex, which is usually covered with hooks or suckers and serves to anchor it to the host.
All of the 5000 or so known species of tapeworms are endoparasites. Most require at least two hosts, with the host of the adult tapeworm a vertebrate. Intermediate hosts are often invertebrates. A number of tapeworm species inhabit humans.
Hickman, C.P. and L. S. Roberts. 1994. Animal Diversity. Wm. C. Brown, Dubuque, IA.
Brusca, R. C., and G. J. Brusca. Invertebrates. 1990. Sinauer Associates, Sunderland, MA.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.