Biochemical and Biophysical Phenomena 763 



ing flashes of longer duration. In the luminous squids and fishes there are 

 luminous organs, perfect lenses, and reflectors to reflect the glow. In the 

 jellyfish {Pelagia noctiluca) the entire surface of the umbrella is covered 

 with glowing granules. In the minute protozoan {Noctiluca miliaris) the 

 luminous granules remain inside the cell. The shrimp emits the luminous 

 granules into the water. Among the plants, certain putrefactive bacteria 

 and special types of mushrooms emit a limited amount of luminescence. 

 Luminous bacteria are frequently found on fish and ham, where they 

 emit rather large quantities of light if the food and oxygen requirements 

 are satisfactory. 



Light afTects animals in several ways. The protozoan, Euglena (Fig. 

 173), has a light-sensitive substance localized in a visible pink spot 

 (stigma) near the reservoir. This mechanism directs the Euglena into 

 the proper light for its metabolic activities. The earthworm has no 

 eyes, yet it moves away from light. This is due to the presence of light- 

 sensitive cells near the surface. The starfish locomotion is influenced by 

 light. Protozoa, Planaria, clams, snails, and certain Crustacea are also 

 influenced by it. The simple eyes of insects and spiders are influenced by 

 light intensities. The compound eye of arthropods is constructed like a 

 bundle of hollow tubes arranged in the form of a cone. The tubes are 

 isolated from each other by black pigment, and together they produce- 

 an upright but reduced image of the object being viewed. The outer 

 end of each tube contains a lens and a facet which is easily seen on the 

 surface of the compound eye. The inner ends of these tubes possess 

 light-sensitive materials connected with the nerves. These compound 

 eyes also give the organism an interpretation of movement of objects. 

 The eyes of vertebrates (Fig. 248) act somewhat like a camera. The 

 lens focuses and forms an image on the retina. A black pigment inside 

 the eyeball is present for the same reason as in a camera. The retina 

 consists of enormous numbers of nerve cells, each with a chemical mate- 

 rial which is temporarily changed by light. Each temporary image on 

 the retina produces chemical changes in the nerve cells varying with 

 the quantity of light on each cell. These chemical changes stimulate 

 other nerve cells which send impulses over the optic nerve to the brain. 



In plants, leaves arrange themselves to get a maximum of light. Sun- 

 light supphes the energy for the chlorophyll of green plants to combine 

 carbon dioxide and water to form carbohydrates through the process of 

 photosynthesis. This process is described in detail in other parts of the 

 book. Parts of plants react to light in diff'erent ways. Stems bend 

 toward light, changing direction with the source of light. Roots bend 



