Salt water covers seventy per cent of the earth's surface: a gigantic challenge to our curiosity and scientific skill.
Marine biologists are slowly unravelling the complex and inter-related life-cycles of the animals in the sea.
Peter Stone introduces the sixth of a series of seven programmes.
With Dr. D.H. Cushing and Dr. F.R. Harden Jones, Fisheries Laboratory, Lowestoft.
(Programme 7: next Thursday, 7.30)