[11] Romanes, _Animal Intelligence_, p. 27.
Darwin tells us that two snails, one of them an invalid, the other in perfect health, lived in the garden of one of his friends. Becoming dissatisfied with their surroundings, the healthy one went in search of another home. When it had found it, it returned and assisted its sick comrade to go thither, evincing toward it, throughout the entire journey, the utmost tenderness and solicitude.[13] The healthy snail must have used its sight, as well as its other senses, to some purpose, for, if my memory serves me correctly, we are told that the sick snail rapidly regained its health amid its new surroundings.
[13] Darwin, _Descent of Man_, pp. 262, 263.
The crayfish also has its eyes at the tips of eyestalks, but the eyes of this creature are very different, indeed, from the eyes of the snail. They are what are known as compound eyes, a type common to the crayfish and lobster families. Viewed from above, the cornea of a crayfish is seen to be divided into a number of compartments or cells, and looks, in this respect, very much like a section of honeycomb. The microscope shows that in each one of these cell-like compartments there is a transparent cone-shaped body; this is called the crystalline cone. The apex of this cone is prolonged into an exceedingly small tube, that enters a striped spindle-like body called the striated spindle; the entire structure is called a visual rod. Nerve-fibrils emanating from the optic nerve enter the striated spindle at its lower extremity, and in this way nervously energize the visual rod. There is a deposit of pigment about the visual rod which arrests all rays of light save those which strike the cornea parallel to the long axis of the crystalline cone. We see from this that the visual picture formed by a crayfish's eye must be made up of many parts; it is, in fact, a mosaic of hundreds of little pictured sections, which, when united, form the picture as a whole. Each visual rod receives its impression from the ray or rays of light reflected from the object viewed which strike it in the line of its long axis; the other rays are stopped by the layer of pigment-cells. When the impressions of all the visual rods are added together, the sum will be a mosaic of the object, but such a perfect one that the junction of its many portions will be absolutely imperceptible.