The three different types of eyes are represented by different types of photoreceptors among invertebrates. They are: 1) eye cup, 2) compound eye, and 3) single-lens eye. The eye cup is a cluster of photoreceptor cells that partially shield adjacent photoreceptor cells. The compound eye consists of many tiny light detectors (photoreceptors). Cray-fish, crabs, and nearly all insects have compound eyes. Single-lens eyes, found in cephalopods such as squids and octopi, are similar to cameras. They have a small opening, the pupil, through which light enters.