Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 86 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. of perfection. In the simplest cases they are known as eye-spots, and consist of irritable protoplasm, , nervous substance, containing pig- ment granules; and in this form they are perhaps scarcely capable of distinguishing light from darkness, but are only susceptible to the warm rays. It is hardly possible to conceive that pigment is indis- pensable for the sensation of light, because there are many eyes of complicated structur
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0101clau Year: 1884 86 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. of perfection. In the simplest cases they are known as eye-spots, and consist of irritable protoplasm, , nervous substance, containing pig- ment granules; and in this form they are perhaps scarcely capable of distinguishing light from darkness, but are only susceptible to the warm rays. It is hardly possible to conceive that pigment is indis- pensable for the sensation of light, because there are many eyes of complicated structure from which pigment may be altogether absent. The view, however, according to which the pigment itself is sensitive to light, , is chemically changed by the light waves and transmits the excitation produced by these movements to the protoplasm or fz Ot Wz FIG. 83—Auditory vesicle of a Heteropod (Pterotrachea). Jf, acoustic nerve ; Of, otolith the fluid of the vesicle ; Wz, ciliated cells on the inner wall of the vesicle ; Hz, auditory cells ; Cz, central cell. the adjacent nervous substance cannot in itself be contradicted, but it is by no means clear that such changes are produced by the light rays as opposed to the heat rays. Of greater importance in this relation appears the special nature of the nerve endings, through which certain movements, progressing in regular waves, the so-called ether waves, are transmitted to the nerve fibres and give rise to a stimulus which travels to the central organ and is by it perceived as light. In all cases in which in the lower animals specific nerve endings cannot be made out, we have probably only to do with a forerunner of the eye, consisting merely of the pigmented termina-
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