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 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. [8] Lubbock, _Senses, Instincts, and Intelligence of Animals_, p. 84.