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What is vision? Why is vision important for life on earth?

Vision is the ability of some living beings to perceive, to distinguish and to interpret luminous stimuli.

Vision is important on earth mainly in the terrestrial and in the superficial aquatic habitats because our planet is intensely exposed to the sunlight and thus light and colors become distinguishing factors of objects present in the environment, even at distance. This distinction provided new survival strategies for the organisms, new protection mechanisms against external dangers, new ways to find food and to communicate with others individuals, new types of courting and reproduction behaviors, etc., i.e., it created new possibilities of interaction with the surrounds and increased capacity to explore new ecological niches.

How  does photosensitivity in cnidarians, annelids and worms differ from insects, cephalopods and vertebrates?

In the first mentioned group of animals there are photoreceptor cells organized in ocelli or diffusely dispersed in the body. These animals do not form images.

In the animals of the second group the photoreceptor cells are part of more sophisticated structures, the eyes, able to form images and to send them to the nervous system.

What are the structures that compose the human vision apparatus?

The organs of the human visual apparatus are the eyes, the optical nerves and the visual areas of the brain (located in the occipital lobes of both hemispheres).