. Principes de zoölogie : toucher la structure, le développement, la distribution, et l'arrangement naturel des races d'animaux, vivant et éteint avec de nombreuses illustrations : partie 1, physiologie comparée : pour l'utilisation des écoles et des collèges. Ing asmall blanc, corps opaque, qui semble être un continuatiopdu nerf optique. Il ne peut pas être douté, donc, thatthese sont des yeux; mais comme ils n'ont pas l'appareil optique qui produit des images, nous devons supposer qu'ils reçoivent canonly une impression générale de la lumière, sans la puissance des objets perspicaces. 86. Les taches oculaires, très similaires à celles de la ne

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. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. ing asmall white, opaque body, which seems to be a continuatiopof the optic nerve. It cannot be doubted, therefore, thatthese are eyes; but as they lack the optical apparatuswhich produces images, we must suppose that they canonly receive a general impression of light, without the powerof discerning objects. 86. Eye-spots, very similarto those of the Nereis, arefound at the extremity of therays of some of the star-fishes, 4in the sea-urchins, at the mar-gin of many Medusse, and insome Polypi. Ehrenberg hasshown that similar spots alsoexist in a large number of theInfusoria. Fig. 17. 87. In all the above-mentioned animals, the eyes, whatever their number, are apart from each other. But there isstill another type of simple eyes, known as aggregate eyes.In some of the millipedes, the pill-bugs, for instance, the eyesare collected into groups, like those of spiders; each eyeinclosing a crystalline lens and a vitreous body, surroundedby a retina and choroH. Such eyes consequently form a 5*. 51 SPECIAL SENSES. natural transition to the compound eyes of insects, to whichwe now give our attention. 88. Compouna, eyes have the same general form asBimple eyes ; they are placed either on the sides of the head, as in insects, or supported on pedestals, as in the crabs.But if Vve examine an eye of this kind by a magnifying lens, we find its surtace to be composed of an infinite number ofangular, usually six-sided faces. If these facettes are re-moved, we find beneath a corresponding number of cones, (c, )side by side, five or six times as long as they are broad, and arranged hke rays around the optic nerve, fromwhich each one receives a little filament, so as topresent, according to Miiller, the following disposition.^«sO^Q^ (^to- ^^0 ^^^^ c

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