The Dawn of Reason | Page 6

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.
[6] Semper, Animal Life, p. 83.
[7] Hickson, The Fauna of the Deep Sea, p. 150 et seq.
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 OEquorea) 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 medus? 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.
[8] Lubbock, Senses, Instincts, and Intelligence of Animals, p. 84.
The eyes of sea-urchins are rather highly developed, having corne?, retin?, 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.
[9] "In Solaster or Asteracanthion the lenses look like brilliant eggs, each in its own scarlet nest."--LUBBOCK, Senses, Instincts, and Intelligence of Animals, p. 132 et seq.
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