Millions of years ago, the horseshoe crab and other marine animals made basie adaptations that ensured their survival. Largely unchanged today, their mechaoisms now help scientists unravel the secrets of the human body. These simple creatures have become important tools for the biomedical researcher. The horseshoe crab has contributed to numerous discoveries in neurobiology, the functioning of antibodies, and the fight against bacterial infections. William Sargent also discusses discoveries made from a number of other marine animals aod fish . The nerves in the squid's mantle, for example, and the bioluminescence of the jellyfish are yielding new findings on neurotransmitters. The research here is presented in the context of a single year, with colorful descriptions of many of these creatures' behaviors and rituals during the seasons . William Sargent, a NOVA consultant, is the author of Shallow Waters: A Year on Cape Cod's Pleasant Bay. He lives on Cape Cod.