This allows the maximal extraction of oxygen possible out of the water. Lungs are endodermal derivations from the posterior pharynx, and so are homologous to the swim bladder. This allows a bony fish to regulate the amount of gas in the swim bladder, depending on the depth the fish is swimming in. The central nervous system of a vertebrate consists of a brain and spinal cord. Some bony fish, e.g. salmon, rely heavily on the sense of smell to find their way back for spawning.
Fish eyes are lidless and lenses are round and firm.
A lateral-line system is seen in fish, where a collection of pits along the side of the fish allow sensation of pressure changes in the water. Fish do this by gills and by kidneys . Imagine the challenge of the lifestyles of eels and salmon, as they spend parts of their life cycles in both salt and freshwater. Cartilaginous fish lay only a few eggs at a time, but bony fish reproduce by producing many, many eggs, with only a few surviving. In a perch, there is a single ovary in the female, but paired testes in the male.