fresh-water Cymothoe living in our own waters that are close kin to the Pelew islander mentioned by Semper, and which are not blind. Along the middle of their backs, over the edge of each segment, there is an oblong dark spot. This little collection of coloring-matter is covered by a transparent membrane, the cornea, and has a special nerve leading to the brain, if I may use the word. These spots are primitive eyes, the analogues of which are preserved by many of the true worms. I am inclined to believe that Semper would find primitive eyes of some form or other in the Cymothoe he mentions, if he were again to examine it. The insects, etc., which dwell in caves, and which have eyes, are new arrivals; they have not dwelt long enough in total darkness to have experienced the full effects of changed surroundings. They show, however, that they are beginning to feel such effects, for there is more or less diminution in the color-cells of the eyes and body coverings. My experiments on fish and frogs show, conclusively, that the color-producing function is directly due to light stimulation. The longer fish and frogs are kept in total darkness, the lower is the number of color-cells and the smaller is the amount of coloring-matter. This accounts for the fact that all animals which have dwelt in darkness for untold ages are absolutely colorless. Pigmented or colored fishes, nevertheless, having well-developed organs of vision, have been taken from such depths (over a mile) as to preclude the possibility of a single ray of daylight.[7] These fishes, however, are phosphorescent, and thus furnish their own light. Moreover, I am inclined to believe that the vast depths of the ocean, in certain localities, lie bathed in a continuous phosphorescent glow, so that creatures living there neither lose their color nor their eyes, sufficient light being present to prevent degeneration. Where eyeless and colorless fishes are brought up from great depths, there the ocean is not phosphorescent, but is in absolute darkness.

The preceding observations indicate that the sense of sight is a very old sense, and that it is to be found in a primitive form (ocelli) in animals of exceedingly low organization. That this is true, I will now attempt to demonstrate.

Sight is the result of the conversion of one form of motion into another—a conservation, as it were, of energy. Thus, waves of light coming from a luminous body are arrested by the pigment-cells of the retina in our eyes and are transmuted into another form of motion, which is called nerve energy (in this instance, sight). It would seem that as far as sight (vision is not included) is concerned, eyes of very simple construction would amply satisfy the needs of thousands of creatures whose existence does not depend upon vision. This supposition is undoubtedly correct; there are many creatures in existence to-day with eyes so exceedingly simple that they can form no visual picture of objects—they are only able to discriminate between light and darkness. Primitive eyes appear in animals very low in the scale of life; probably the most remarkable of these early organs of sight are to be found in the medusa, or jelly-fish. This creature, with its bell-shaped body and pendent stem, bears a striking resemblance to an umbrella; noting this resemblance, naturalists have given the name manubrium, "handle," to the stem. Around the edge of the umbrella, and situated at regular intervals, are certain round, cell-like organs, which vary considerably in number. Some species have only eight, while others have sixty, eighty, and even (in Œquorea) as high as six hundred.[8] These so-called "marginal bodies" are the eyes of the jelly-fish. By many biologists these organs are considered to be ears; they contain within their capsules transparent bodies, which some scientists deem otoliths, or "hearing-stones." Experimentation and microscopical examinations, however, have taught me very recently to believe otherwise. In these marginal bodies there is always a deposit of pigment; this is, unquestionably, a primitive retina, while the transparent disk is, indubitably, a primitive lens. That these creatures can tell the difference between light and darkness is a fact easily demonstrated. Time and again have I made them follow a bright light around the wall of the aquarium in which they were confined. On one occasion I made some medusæ tipsy, and their drunken gravity as they rolled and staggered through the water in pursuit of the light was as sorrowful as it was instructive; their actions in this respect were those of intoxicated men. After I had siphoned off the alcoholized water and replaced it with pure, they rapidly regained their normal status; whether or not any of them felt any evil effects from their involuntary debauch, I am not prepared to state.

The eyes of sea-urchins are rather highly developed, having corneæ, retinæ, and lenses. The lens generally lies in a mass of pigment, and, as Lubbock remarks, looks like a brilliant egg in a scarlet nest.[9] The eyes are scattered over the dorsal surface of the creature's body, and are commonly situated just beneath the skin; they are, however, sometimes elevated on pear-shaped bulbs. The eyes of starfish are generally quite primitive in character, as far as I have been able to determine, being simply pigmented spots which are supplied with nerves; in several species, however, I have been able to make out lenses. The eyes are arranged along the rays or arms, and vary in number.

Even the stay-at-home and humble oyster has eyes (not the round, fleshy muscle called the "eye" by gourmands and epicures, but bright spots around the edge of the mantle)—primitive eyes, it is true, yet amply sufficient for the needs of a domestic, non-travelling, home body like the oyster.

In most of the worms the eyes are simple ocelli—spots of pigment supplied with nerves. These eyes can discriminate between light and darkness, which is all that is required of them; but in the Alciope, a small sea-worm, these organs are brought to a high degree of perfection. This worm is exceedingly transparent, so that when observing it, it is difficult to make out more than its large orange eyes and the violet segmental organs of each ring. It looks like an animated string of violet disks surmounted by a pair of orange-colored eyeglasses. The eye of this creature is strikingly like that of a human being; it has a cornea, an "eye-skin," a lens, vitreous humor (posterior chamber), and retina.

Another aquatic worm, Myrianida, is still more remarkable, not only on account of its eyes, but also on account of the wonderful way in which it reproduces its young. When seen swimming in the water it presents the appearance of a long, many-ringed worm, which impels itself through and by the aid of its hundreds of flat, oar-like legs. Closer inspection reveals the startling fact that this seemingly single worm is really a multiple worm—six or more individuals being joined together, thus forming a living chain. This creature reproduces itself by fissigemation; that is, when the young worms arrive at a certain age they separate from the parent worm and begin life as individuals. These in turn eventually become multiple worms and divide into individuals, and so on ad infinitum. The tail worm, or that section farthest from the head, is the oldest and is always the first to leave its comrades and take up a separate existence.