What organ does cartilaginous fish lack?

Cartilaginous fish are vertebrates. They have many similarities with their closely related bony fish. But there are also some differences. Bony fish have a swim bladder. It helps them stay at a certain depth of the reservoir and not sink to the bottom. Cartilaginous fish simply do not have this organ. In order to be at a certain depth, they need to constantly move their fins. If we draw an analogy, then bony fish, like airships, can be at a certain height. The cartilaginous ones resemble a helicopter, since the blades must be in constant motion.