. Animal studies. THE SPECIAL SENSES 385 The simplest eyes, if we may call them eyes, are not capable of forming an image or picture of external objects. They only make the animal better capable of distinguish- .ing between light and darkness or shadow. Many lowly organized animals, as some polyps, and worms, have certain cells of the skin specially provided with pigment. These cells grouped together form what is called a pigment fleck, which can, because of the presence of the pigment, absorb more light than the skin cells, and are more sensitive to the light. By such pigment-flecks, or eye-s
. Animal studies. THE SPECIAL SENSES 385 The simplest eyes, if we may call them eyes, are not capable of forming an image or picture of external objects. They only make the animal better capable of distinguish- .ing between light and darkness or shadow. Many lowly organized animals, as some polyps, and worms, have certain cells of the skin specially provided with pigment. These cells grouped together form what is called a pigment fleck, which can, because of the presence of the pigment, absorb more light than the skin cells, and are more sensitive to the light. By such pigment-flecks, or eye-spots, the animal can detect, by their shadows, the passing near them of mov- ing bodies, and thus be in some measure informed of the approach of enemies or of prey. Some of these eye-flecks are provided, not simply with pigment, but with a simple sort of lens that serves to concentrate rays of light and make this simplest sort of eye even more sensitive to changes in the in- tensity of light (Fig. 236). Most of the many - celled ani- mals possess eyes by means of which a picture of exter- nal objects more or less nearly complete and perfect can be formed. There is great variety in the finer structure of these picture-forming eyes, but each consists essentially of an inner delicate or sensitive nervous surface called the retina, which is stimulated by light, and is connected with the brain by a large optic nerve, and of a transparent light- refracting lens lying outside of the retina and exposed to the light. These are the constant essential parts of an image - forming and image-perceiving eye. In most eyes there are other accessory parts which may make the whole. Fig. 236.—The simple eye of a jelly-fish (Lizzia koellikeri).—After O. and K. 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 Jordan,
Size: 2130px × 1173px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorjordanda, bookcentury1900, bookdecade1900, bookyear1903
