Don't let the bedbugs bite! Actually, if you were to worry about all of the parasites living on your body (sometimes helping you out), you'd never fall asleep in the first place. As a living organism part of a bigger ecosystem, humans play host to unumerable other living things. This Topic In Depth offers a look into the fascinating world of Human Ectoparasites. The first stop -- a page from...

This Web site from the Canadian Lung Association contains an excellent collection of learning resources about the respiratory system for grades 1-12. Content varies according to grade range; for instance, the section for grades 1-3 covers basics of lungs and their function with an illustrated story, coloring book, interactive games, and downloadable worksheets. Older students focus in on smoking,...

This collection of Web sites about twins and twin biology starts with a short feature from The Straight Dope (1), which explains the phenomenon of superfecundation. Next is a more extensive piece from Scientific American (2), offering an interesting look at the role genetics may play in the chance of having fraternal twins. The following Web site contains a brief MSNBC story on the recent findings...

Understanding Life is the educational website of The Physiological Society, providing "support for the teaching and learning of physiology." A good place to start is the What is Physiology? area, which offers an overview of this field of human inquiry. The Resources area is a well-designed archive of instructional materials that include "The story of a single heartbeat," "The Science of Life," and...