The medusa is a tiny jellfish that lives on the ventral surface of a sea slug found in the Bay of Naples. Readers will find themselves caught up in the fate of the medusa and the snail as a metaphor for eternal issues of life and death as Lewis Thomas further extends the exploration of a man and his world begun in ''The Lives of a Cell.'' Among the treasures in this magnificent book are essays on the human genius for making mistakes, on disease and natural death, on cloning, on warts, and on Montaigne, as well as an assessment of medical science and health care. In these essays and others, Thomas once again conveys his observations of the scientific world in prose marked by wonder and wit.