Wow--how flatworm caccoon embryos survive deep sea conditions,

What was inside? The secret of the black cocoons

Although the structures were about 3 millimetres in diameter, they were not actually eggs, but leathery egg capsules (known as cocoons). When Dr Keiichi Kakui from Hokkaido University opened the cocoons using a microscope, each cocoon produced a milky liquid (later identified as yolk) that leaked out. Each egg capsule contained between three and seven flatworm embryos, with some of the embryos already showing signs of developed internal organs, according to the research archived by the Royal Society Publishing.