. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. MULTICELLULAR ANIMALS 225 in Man is much less than in many other vertebrates. The taste buds are located chiefly on the tongue in the human adult but in the very young they are also found elsewhere in the mouth. Light Receptors. The receptfon of a stimulus by light pre- supposes the presence in the animal of some sort of substance that is light-sensitive, affected chemically by light rays. In certain Proto- zoa, for example euglena, the light-sensitive substance is localized in a visible spot
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. MULTICELLULAR ANIMALS 225 in Man is much less than in many other vertebrates. The taste buds are located chiefly on the tongue in the human adult but in the very young they are also found elsewhere in the mouth. Light Receptors. The receptfon of a stimulus by light pre- supposes the presence in the animal of some sort of substance that is light-sensitive, affected chemically by light rays. In certain Proto- zoa, for example euglena, the light-sensitive substance is localized in a visible spot, the stigma, seen as a pink body near the reservoir (Fig. 29). This spot must not be regarded as an eye similar to the PIGMENT NEftVE FIBERS. COffNEA CRYSTALLINE CONE TAPETUM CELL Fig. 153.—Diagram of the compound e^^e of an insect. (Partly after Kiihn: Grun- driss der cdlgemeinen Zoologie. Georg Theime, Leipzig.) image-forming structures of higher animals, but merely as a region of light sensitivity. In the eyes of invertebrates the light-sensitive substances are located in association with the endings of nerves. In some forms structures occur that have the properties of convex lenses and converge light on these endings. In insects and other Arthropoda the eyes are compound, consisting of a considerable number of tubes shaped like inverted cones. In the apex of each cone is a nerve ending, with associated light-sensitive structures; the base of each cone contains a lens which serves to converge light rays on the apex (Fig. 153). It is said that this type of eye, by reason of the mosaic image which it forms, is peculiarly suited to the detection of Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Buchanan, James William, 1888-. New York, London, Harper & brothers
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