. 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 Ne
. 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 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 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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