The adverb always in the above sentence is, strictly speaking, not exactly accurate, for on one occasion I saw the separation occur at the second head from the tail, thus producing twins. The two sections came apart, however, in a very few seconds after their departure from the colony. I am inclined to believe that this deviation from the normal was due to accident; probably to manipulation. This annelid is really "many in one" until the very moment of division; the alimentary canal, nerves, blood-vessels, etc., extend in unbroken continuity from the head of the parent worm to the tail of the last section. In every fourth (sometimes fifth) ring two round, dark-colored spots will be observed; these spots are ocelli, and some of them eventually become the eyes of young worms. These organs even in their embryonic state possess sight, for they have special nerves and pigment-cells; they can differentiate between light and darkness.

The snail carries its eyes in telescopic watch-towers. This animal is, for the most part, nocturnal in its habits, and, since prominent and commanding view points are assigned to its organs of sight, one would naturally expect to find a comparatively high degree of development in them; and this supposition is correct. The eyes of the creature are in the extreme tips of its "horns," and consist of (1) a cornea, (2) a lens, and (3) a retina. Lubbock is rather disposed to decry the visual powers of the snail;[10] my conclusions, drawn from personal observations, are, however, directly the opposite. The position of the eyes at the extreme tips of the horns naturally indicates that they subserve a very useful purpose; otherwise they would not have attained such prominence and such a high degree of development. Actual experimentation declares that the garden snail can see a moving white object, such as a ball of cotton or twine, at a distance of two feet. In my experiments I used a pole ten feet in length, from the tip of which a white or dark ball was suspended by a string. The ball was made to describe a pendulum-like movement to and fro in front of the snail on a level with the tips of its horns. Time and again I have seen a snail draw in its horns when it perceived the white ball, to it an unknown and terror-inspiring object. I have likewise seen it change its line of march, and proceed in another direction, in order to avoid the mysterious white stranger dancing athwart its pathway. Dark-colored objects are not so readily perceived; at least, snails do not give any evidence of having seen them until they are brought within a foot of the creatures under observation. A snail will generally see a black ball at twelve or fourteen inches; sometimes it will not perceive the ball, however, until it has been brought to within six or eight inches of its eyestalks. During the season of courtship snails easily perceive one another at the distance of eighteen or twenty inches. I have often watched them at such times, and have been highly entertained by their actions. The emotional natures of snails, as far as love and affection are concerned, seem to be highly developed, and they show plainly by their actions, when courting, the tenderness they feel for each other. This has been noticed by many observers of high authority, notably Darwin, Romanes, and Wolff.[11] Mantagazza, a distinguished Italian scientist, in his Physiognomy and Expression, writes as follows: "As long as I live I shall never see anything equal to the loving tenderness of two snails, who, having in turn launched their little stone darts (as in prehistoric times), caress and embrace each other with a grace that might arouse the envy of the most refined epicurean."[12]

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.

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.

The crayfish can see quite well. It has been thought that this creature uses its sense of smell more than its sense of sight in the procurement of its food. This is undoubtedly true where the animal is surrounded by water that is muddy, or that is otherwise rendered opaque. The odoriferous particles coming from the food being carried to the creature by the water, it follows them until it arrives at this source.

It is different, however, in clear water and on land. I have seen crayfish rush down stream after bits of meat thrown to them, thus showing that here, at least, the sense of sight directed them. Again, I have enticed crayfish from clear streams by slowly dragging a baited hook in front of them. Moreover, when high and dry on land, I have seen them follow with their eyes and bodies the tempting morsel as it waved to and fro in the air above their heads.

The female crayfish carries her eggs beneath her tail, and, when they have hatched out, the young find this sheltering member a safe and cosey dwelling-place until they have grown strong enough to enter life's struggle. At such times, the mother crayfish is quite brave, and will do battle with any foe. With her eyestalks protruded to their utmost extent, she vigilantly watches her enemy. Her eyes follow his movements, and her