The head of the common house fly is dominated by a pair of large compound eyes containing approximately 4,000 image-forming elements called ommatidia. Each cone-shaped ommatidium consists of a lens, a crystalline rod, and a collection of light-sensitive cells. Compound eyes are only found in arthropods such as insects and crustaceans.
The simplest animal eyes occur in the cnidarians and ctenophores, phyla comprising the jellyfish and somewhat similar primitive animals. These eyes, known as pigment eyes, consist of groups of pigment cells associated with sensory cells and often covered with a thickened layer of cuticle that forms a kind of lens. Similar eyes, usually having a somewhat more complex structure, occur in worms, insects, and mollusks.
Two kinds of image-forming eyes are found in the animal world, single and compound eyes. The single eyes are essentially similar to the human eye, though varying from group to group in details of structure. The lowest species to develop such eyes are some of the large jellyfish. Compound eyes, confined to the arthropods, consist of a faceted lens, each facet of which forms a separate image on a retinal cell, creating a moasic field. In some arthropods the structure is more sophisticated, forming a combined image.
The eyes of other vertebrates are essentially similar to human eyes, although important modifications may exist. The eyes of such nocturnal animals as cats, owls, and bats are provided only with rod cells, and the cells are both more sensitive and more numerous than in humans. The eye of a dolphin has 7000 times as many rod cells as a human eye, enabling it to see in deep water. The eyes of most fish have a flat cornea and a globular lens and are hence particularly adapted for seeing close objects. Birds’ eyes are elongated from front to back, permitting larger images of distant objects to be formed on the retina.