This second collection of short pieces by the President of Memorial Sloan-Kettering Cancer Center will bring joy to those who delighted in Lives of a Cell. The title essay concerns a jellyfish (the medusa) and a sea slug (the snail) who live in happy symbiosis in the Bay of Naples. Their ability to find each other is Thomas' springboard for a discussion of the notions of "self" and "other"--of biochemical recognition whether manifest in attachments between discrete organisms or in the distinction between foreign and self in the immune system. The medusa-snail story is a particularly extraordinary tale of a mature jellyfish engulfing a tiny newly-hatched slug--only to be devoured bit by bit until the snail dominates and the jellyfish is reduced to a round "successfully edited parasite" affixed to the skin near the snail's mouth. Thomas' unexpected turns of phrase and love of words and their origins is revealed again and again in essays ranging in subject from our present zeal for dying gracefully to how-not-to-choose students for medical schools. The origin of hubris in hybrid--via us, meaning out, and gwer, meaning violence and strength--is the fulcrum on which turns a telling piece which should dispose of all who'd set limits on scientific research. The celebrated essay "On Transcendental Metaworry"--in which Thomas wryly dispatches the whole kit and kaboodle of instant paths to enlightenment--is a rondo of variants of the origins of "worry." To be sure, there are dispensable pieces--a putdown of electronic music, a gratuitous reminder of how little physicians earned 50 years ago--but this is to quibble. Read Thomas for his estimable style--often disarmingly simple, even colloquial--and the wit and insight into life and medicine his writing embodies.