Stages in the evolution of eyes among molluscs. Source: Wikipedia. a: Flat eye; b: cup eye; c: pinhole eye; d: vesicular eye; e: lens eye.

The example of the molluscs offers a good opportunity to observe the evolution of light sense organs in the animal kingdom. Among the numerous and various groups of molluscs there are primordial and advanced, movable and sessile species.

In the most primitive form light perception happens by single sense cells located somewhere in the body. Singular light sense cells dispersed over the body surface, as on snails and segmented worms, can tell the difference between light and dark, so the animal may benefit from a shadow reflex to protect itself against predators. They are, however, not a sense organ in the common sense of speaking: A sense organ is a complete organ, not just singular cells, specialized in a defined sensory performance. The first light sense organ is a specialized field of light sense cells and pigment cells for
lateral isolation. It is called a flat eye. It enables its possessor to differentiate between light and dark, but only basically makes it possible to tell where the light comes from.

Flat eyes today can still be found in primitive groups of invertebrates, such as jellyfish (Coelenterata). It may also be assumed that the molluscs' ancestors, primitive, worm-like ground-living creatures, also possessed such flat eyes.

A primitive flat eye may be of valuable use to an animal either sessile or moving passively. The directed movement of more highly developed molluscs required the formation of more advanced light sense organs. In the consequence the light-sensitive epithelium of the flat eye caved in to form a pit. So the light sense cells on facing sides of the eye can tell apart light and shade. That makes it possible to determine where the light comes from. Pit shaped eyes can be found in sessile and slow moving invertebrates.   ...

While a pit eye may be able to differentiate between light and shade, it is not capable of producing pictures. Especially for predatory molluscs, having to observe and to follow their prey, an improvement of the eye's picture projection capability was necessary: The eye opening narrowed, and in consequence the picture projected on the retina became more focused. So the pigmented cup eye came into existence. Today, in its primitive state, this type of eye can be found among certain bivalves and turbellarian worms.

Pigmented cup eyes can also be found among primitive, mainly sessile, gastropods, such as limpets (Patellidae) and ormers (Haliotidae).

In the further course of evolution, the eye opening reduced in size and as a result the eye achieved abilities comparable to a so-called pinhole camera: A focused, but low-light picture can be projected to the retina. Among the molluscs, pinhole eyes can be found among primitive cephalopods, such as Nautilus. Nautilus is a living fossil, a remnant from the Mesozoic. It is also assumed, that fossil cephalopods, such as the giant endocerate Cameraceras from the Ordovician had comparable eyes.  ...