Final answer:
Gills ceased to develop in vertebrates other than fish because the function of gills shifted from filtering food to absorbing oxygen. As vertebrates evolved other adaptations for breathing air, the need for gills diminished.
Step-by-step explanation:
In the evolution of vertebrates, gills ceased to develop in animals other than fish because the function of gills shifted from filtering food to absorbing oxygen.
Gills provide a large surface area for oxygen uptake, allowing fish to grow larger and more active.
However, as vertebrates moved onto land and evolved other adaptations, such as lungs for breathing air, the need for gills diminished.