A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-lensed "compound eye", and is not necessarily at all simple in the usual sense of the word. The structure of an animal's eye is determined by the environment in which it lives, and the behavioural tasks it must fulfill to survive. Arthropods differ widely in the habitats in which they live, as well as their visual requirements for finding food or conspecifics, and avoiding predators. Consequently, an enormous variety of eye types are found in arthropods. They possess a wide variety of novel solutions to overcome visual problems or limitations. Use of the term simple eye is flexible, and must be interpreted in proper context; for example, the eyes of humans and of other large animals such as most cephalopods, are camera eyes and in some usages are classed as "simple" because a single lens collects and focuses light onto the retina (analogous to film). By other criteria, the presence of a complex retina distinguishes the vertebrate camera eye from the simple stemma or ommatidium. Additionally not all ocelli (ocellus in singular) and ommatidia of invertebrates have simple photoreceptors; many, including the ommatidia of most insects and the central eyes of Solifugae have various forms of retinula, and the Salticidae and some other predatory spiders with seemingly simple eyes, emulate retinal vision in various ways. Many insects have unambiguously compound eyes consisting of multiple lenses (up to tens of thousands), but achieve an effect similar to that of a camera eye, in that each ommatidium lens focuses light onto a number of neighbouring retinulae. Ocelli or eye spots Some jellyfish, sea stars, flatworms, and ribbonworms have the simplest "eyes" – pigment spot ocelli – which have randomly distributed pigment, and which have no other structure (such as a cornea, or lens). The apparent "eye color" in these animals is red or black. However, box jellyfish have more complex eyes, including those of which have distinct retina, lens, and cornea. Many snails and slugs (gastropod mollusks) also have ocelli, either at the tips, or at the bases, of their tentacles. However, some other gastropods, such as the Strombidae, have much more sophisticated eyes. Giant clams (Tridacna) have ocelli