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 larve 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. 
