Complex sentence: The crystalline lenses of the fishes' eyes are extremely convex, almost spherical, and its refractive index is highest of all the animals which enables proper focusing of the light rays and in turn proper image formation on the retina.
Split into simpler sentences: The crystalline lenses of fishes' eyes are extremely convex, almost spherical, and their refractive indices are the highest of all the animals. These properties enable proper focusing of the light rays and in turn proper image formation on the retina.