SIGHT IN INVERTEBRATES I57 



body causes the surface membrane of the cell to bulge out- 

 wards slightly so that, especially in non-aquatic animals, 

 light falling upon it is refracted and concentrated upon the 

 pigment. Such simple eyes, like eye-spots, are light-gather- 

 ing organs and do not form images, but the basic structures 

 are present for the development of image-forming organs 

 by further stages of evolution. In the first stage, still a light- 

 gatherer and not an image-former, the density of the 

 refracting part of the cell is increased, thereby producing 

 a very simple lens. In the next stage the cell is divided into 

 two, so that a lens-cell lies above a retinular-cell containing 

 the pigment. This simplest form of eye containing an optical 

 system is found in the young stages of some Ascidians or 

 Sea-squirts, animals that swim freely in the sea while they 

 are minute larvae but then settle on the bottom and become 

 superficially more like vegetables in appearance when adult. 

 Once a lens-cell and a retinular-cell are separated the further 

 stages of evolution to produce an image-forming eye of great 

 efficiency are merely those of an increased differentiation of 

 cell structure, an enormous increase in cell numbers (the 

 human eye is said to contain 137,000,000 nerve endings) and 

 a general increase in complexity. 



A series of eyes ascending from the simplest to those 

 probably as efficient as our own, perhaps even more so, 

 can be traced in the molluscs, the shell-fish that include 

 the snail, limpet and periwinkle, the oyster, clam and cockle, 

 the squids and octopuses — very different from those other 

 shell-fish, the Crustacea, which include crabs, lobsters and 

 shrimps. 



The spiral-shelled molluscs, which creep about by means 

 of the "foot" on the under-surface of the body and have a 

 definite head-end, usually have a pair of eyes, although 

 many species that burrow in mud and sand, or live in very 

 deep water where it is dark, have no eyes ; in some, such as 

 the snail, the eyes lie at the ends of a pair of tentacles. In 

 the limpet the eyes are merely little pits in the skin; they 

 are lined with pigment and retina-cells, and are open to 

 the sea-water which bathes the surface of the lining cells — 

 the lens and all the other usual eye components are missing. 

 Such an eye is obviously no more than a light-gatherer. 



