Reef-dwelling catsharks wrap their fertilised eggs in leathery box-shaped cases commonly called "mermaid's purses". The tough, collagenous wall of the purse protects the developing young until it's ready to hatch -- anywhere from a month to a year after it's laid, depending on its species.
This short video from the Smithsonian Marine Station in Fort Pierce, Florida gives us a peek inside the case. The shark embryo is swimming! There's a reason for that: It needs oxygen.
The enormous yolk attached to its umbilical cord gives the embryo all the food it needs to grow. But once the embryo grows big enough, it needs more oxygen than the water inside the case can hold. And since the purse is tough and waterproof, it's hard for fresher water to find its way inside.
In some catshark species, a 'swim-based' pump solves the problem. Although the purse is sealed early in development, tiny holes eventually open on the long tendrils at its ends, and the embryo starts to swim. Its motion pumps water through the case, drawing oxygen-rich water through one end and pushing waste-filled water out the other.