Final answer:
The layer of cells that improves the visual acuity of nocturnal animals is the tapetum lucidum.
Step-by-step explanation:
The layer of cells that lie behind the retina and improve the visual acuity of nocturnal animals is called the tapetum lucidum. The tapetum lucidum is a layer of reflective cells that sit behind the retina in the eyes of certain animals, such as cats and dogs. It helps to enhance their night vision by reflecting light back through the retina, giving the photoreceptor cells a second chance to detect incoming light.