THE SPECIAL SENSES 237 



these sounds are so feeble or so high in pitch that they are 

 rarely heard by us. Certain butterflies make an odd click- 

 ing sound, as do some of the water-beetles. In Japan, 

 where small things which are beautiful are prized not less 

 than large ones, singing insects are kept in cages and 

 highly valued, so that their capture becomes a lucrative 

 industry, just as it is with song birds in Europe and Amer- 

 ica. Among the many species of Japanese singing insects 

 is a night cricket, known as the bridle-bit insect, because 

 its note resembles the jingling of a bridle-bit. 



124. The sense of sight. ^Not all animals have eyes. 

 The moles which live underground, insects, and other ani- 

 mals that live in caves, and the deep-sea fishes which live 

 in waters so deep that the light of the sun never comes 

 to them, have no eyes at all, or have eyes of so rudimentary 

 a character that they can no longer be used for seeing. 

 But all these eyeless animals have no eyes because they 

 live under conditions where eyes are useless. They have 

 lost their eyes by degeneration. There are, however, many 

 animals that have no eyes, nor have they or their ancestors 

 ever had eyes. These are the simplest, most lowly organ- 

 ized animals. Many, perhaps all eyeless animals are, how- 

 ever, capable of distinguishing light from darkness. They 

 are sensitive to light. An investigator placed several indi- 

 viduals of the common, tiny fresh- water polyp (Hydra) in a 

 glass cylinder the walls of which were painted black. He 

 left a small part of the cylinder unpainted, and in this part 

 of the cylinder where the light penetrated the Hydras all 

 gathered. The eyeless maggots or larvae of flies, when 

 placed in the light will wriggle and squirm away into dark 

 crevices. They are conscious of light when exposed to it, 

 and endeavor to shun it. Most plants turn their leaves 

 toward the light ; the sunflowers turn on their stems to 

 face the sun. Light seems to stimulate organisms whether 

 they have eyes or not, and the organisms either try to get 

 into the light or to avoid it. But this is not seeing. 



