Another utilitarian entry in the series which includes Jacobson's introductions to shells, sponges, and starfish. The classification, structure, and life stages of this peculiar animal are duly noted, as are the workings of its nerve cell network and its methods of stinging prey and moving about (mostly, they just drift); then some individual jellyfish are surveyed, beginning with ""Harmful"" ones--the Portuguese man-o-war (called an animal but ""just as accurately described as a colony of polyps""), the deadly Australian sea wasp that can and does kill people. ""Useful"" jellyfish are harder to find, though the authors point out that some small fish take shelter in their tentacles and octopuses have been known to appropriate their stingers. Also, some jellyfish are used in medicine, but this is barely mentioned though readers might wish to know more about the ""anticancer agent"" said to have been ""recently discovered"" in several species. The fascinating topic of bioluminiscence is similarly slighted, and the authors end with the dubious suggestion that readers make pets of jellyfish, even though it's hard to keep them alive.