A Mermaid's purse is the egg sac from species of dogfish, skates and rays. They are leathery, square or rectangular objects, up to 9cm in length depending on species, with strings or tendrils at each corner - people used to think they looked like handbags or purses. They are usually dark brown or black and may be found, occasionally still containing a young fish embryo, washed up on the beach.
When these fish mate, sperm is inserted into the belly of the female and the eggs are fertilized internally. Each egg sac contains a single egg, and sperm enter in the water which flows through tiny holes in the corners of the sac. A few purses are depostited every few days onto seaweed where the mother attaches them by winding by the tendrils around the weed. The embryos develops inside the purse and when they are large enough the young fry can be seen wriggling about inside. After a few weeks the young fish emerge through splits in the side of the egg cases and swim away.